Problem:
Score: 300 points

Problem Statement

We have a canvas divided into a grid with H rows and W columns. The square at the i-th row from the top and the j-th column from the left is represented as (i, j).

Initially, all the squares are white. square1001 wants to draw a picture with black paint. His specific objective is to make Square (i, j) black when s_{i, j}= #, and to make Square (i, j) white when s_{i, j}= ..

However, since he is not a good painter, he can only choose two squares that are horizontally or vertically adjacent and paint those squares black, for some number of times (possibly zero). He may choose squares that are already painted black, in which case the color of those squares remain black.

Determine if square1001 can achieve his objective.

Constraints

H is an integer between 1 and 50 (inclusive).

W is an integer between 1 and 50 (inclusive).

For every (i, j) (1 \leq i \leq H, 1 \leq j \leq W), s_{i, j} is # or ..

Input

Input is given from Standard Input in the following format:

H W
s_{1, 1} s_{1, 2} s_{1, 3} ... s_{1, W}
s_{2, 1} s_{2, 2} s_{2, 3} ... s_{2, W}
: :
s_{H, 1} s_{H, 2} s_{H, 3} ... s_{H, W}

Output

If square1001 can achieve his objective, print Yes; if he cannot, print No.

Sample Input 1

3 3
.#.
###
.#.

Sample Output 1

Yes

One possible way to achieve the objective is shown in the figure below. Here, the squares being painted are marked by stars.

Sample Input 2

5 5
#.#.#
.#.#.
#.#.#
.#.#.
#.#.#

Sample Output 2

No

square1001 cannot achieve his objective here.

Sample Input 3

11 11
...#####...
.##.....##.
#..##.##..#
#..##.##..#
#.........#
#...###...#
.#########.
.#.#.#.#.#.
##.#.#.#.##
..##.#.##..
.##..#..##.

Sample Output 3

Yes

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

//using static System.Linq.Enumerable;
using static System.Console;
using static AtCoder.Util;
using static AtCoder.Cin;
using static System.Math;
using static AtCoder.MyMath;
using static AtCoder.StreamExtensions;
using static AtCoder.Matrix;

namespace AtCoder {
    using VI = VectorInt2;

    static class Program {
        static void Main() {
            var h = ReadInt();
            var w = ReadInt();
            var drawing = h.Times(ReadString).ToList();

            Func<int, int, bool> isBlack = (i, j) => {
                if (i < 0 || h <= i || j < 0 || w <= j) return false;
                return drawing[i][j] == '#';
            };

            for (int i = 0; i < h; i++) {
                for (int j = 0; j < w; j++) {
                    if (drawing[i][j] == '.') continue;
                    if (isBlack(i, j - 1) || isBlack(i, j + 1) || isBlack(i - 1, j) || isBlack(i + 1, j)) continue;

                    "No".WriteLine();
                    return;
                }
            }
            "Yes".WriteLine();
            return;
        }
    }
}
/* ***************** Following Contents are my common library ******** */

namespace AtCoder {
    static class Algorithm {

        public static LongRange BinSearch(LongRange range, Func<long, bool> isBigger, long width) => BinSearch(range, isBigger, x => range.Right - range.Left < width);
        public static LongRange BinSearch(LongRange range, Func<long, bool> isBigger, Func<LongRange, bool> stopCondition) {
            if (stopCondition(range)) return range;
            var middle = range.Left + (range.Right - range.Left) / 2;
            return isBigger(middle) ? BinSearch(new LongRange(range.Left, middle), isBigger, stopCondition)
                : BinSearch(new LongRange(middle, range.Right), isBigger, stopCondition);

        }
    }

    static class MyMath {

        public static long GCD(long a, long b) =>
            a < b ? GCD(b, a)
            : b > 0 ? GCD(b, a % b)
            : a;
        public static long GCD(this IEnumerable<long> source) => source.Aggregate(GCD);

        public static long LCM(long a, long b) {
            var g = GCD(a, b);
            var a1 = a / g;
            var b1 = b / g;
            return g * a1 * b1;
        }
        public static long LCM(this IEnumerable<long> source) => source.Aggregate(LCM);

        public static long Factorial(this long n) => Range(1, n).Aggregate(1L, Multiply);
        public static long nPr(int n, int r) => r < 0 || r > n ? 0 : FromTo(n - r + 1, n).Select(x => (long) x).Aggregate(1L, Multiply);
        public static long nCr(int n, int r) => nPr(n, r) / Factorial(r);

        public static long Inc(long i) => i + 1;
        public static long Dec(long i) => i - 1;
        public static long Plus(long i, long j) => i + j;
        public static Func<long, long> Plus(long j) => i => i + j;
        public static long Minus(long i, long j) => i - j;
        public static Func<long, long> Minus(long j) => i => i - j;
        public static long Multiply(long i, long j) => i * j;
        public static Func<long, long> Multiply(long j) => i => i * j;
        public static long Divide(long i, long j) => j / j;
        public static Func<long, long> Divide(long j) => i => i * j;
        public static long Mod(long i, long j) => i % j;
        public static Func<long, long> Mod(long j) => i => i % j;

        public static long Max(params long[] ns) => ns.Max();

        public static long LowerBound(this long m, long lbound) => Max(m, lbound);
        public static long UpperBound(this long m, long ubound) => Min(m, ubound);
        public static long Bound(this long m, long lowerBound, long UpperBound) => m.LowerBound(lowerBound).UpperBound(UpperBound);

        public static long LowerBound(this int m, long lbound) => Max(m, lbound);
        public static long UpperBound(this int m, long ubound) => Min(m, ubound);
        public static long Bound(this int m, long lowerBound, long UpperBound) => m.LowerBound(lowerBound).UpperBound(UpperBound);

        public static bool InRange(this long x, long min, long max) => min <= x && x <= max;
        public static bool IsEven(this int x) => x % 2 == 0;
        public static bool IsOdd(this int x) => x % 2 != 0;
        public static bool IsEven(this long x) => x % 2 == 0;
        public static bool IsOdd(this long x) => x % 2 != 0;

        public static IEnumerable<int> ToDigits(this long n) =>
            n.ToString().Select(x => x.ToInt());
        public static IEnumerable<int> ToDigits(this int n) => ((long) n).ToDigits();

        public static IEnumerable<int> Factors(this int n) {
            for (int i = 1; i <= n; i++)
                if (n % i == 0) yield return i;
        }

        public static IEnumerable<long> Factors(this long n) {
            for (long i = 1; i <= n; i++)
                if (n % i == 0) yield return i;
        }

        public static IEnumerable<int> Natural() {
            for (int i = 0;; i++) yield return i;
        }
        public static IEnumerable<int> Positive() => Natural().Skip(1);

        public static long Pow(this long i, long exp) => (exp == 0) ? 1 : i * Pow(i, exp - 1);
        public static long Pow(this int i, long exp) => (exp == 0) ? 1 : i * Pow(i, exp - 1);

        public static int IntLog(this long n, long baseNum) {
            int count = 0;
            while (n > 1) {
                n /= baseNum;
                count++;
            }
            return count;
        }

    }

    static class Util {
        public static InvalidOperationException UnreachableException = new InvalidOperationException("unreachable.");

        public static string Show(this IEnumerable<char> source) => new string(source.ToArray());
        public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source) => new HashSet<T>(source);

        public static MultiSet<T> ToMultiSet<T>(this IEnumerable<T> t) => new MultiSet<T>(t);

        public static bool IsPalindrome(string s) {
            for (int i = 0; i < s.Length / 2; i++) {
                if (s[i] != s[s.Length - 1 - i]) return false;
            }
            return true;
        }
        public readonly static VectorInt2[] Dir8 = {
            new VectorInt2(-1, -1),
            new VectorInt2(-1, 0),
            new VectorInt2(-1, 1),
            new VectorInt2(0, -1),
            new VectorInt2(0, 1),
            new VectorInt2(1, -1),
            new VectorInt2(1, 0),
            new VectorInt2(1, 1)
        };

        public const long MODCONST = 1000000007;

        public static T debug<T>(this T value) {
            Console.Error.WriteLine($"debug:{value}");
            return value;
        }

        public static T Id<T>(T t) => t;
        public static long ToLong(this String s) => long.Parse(s);
        public static long ToLong(this char c) => c - '0';
        public static int ToInt(this String s) => int.Parse(s);
        public static int ToInt(this char c) => c - '0';

        public static VectorInt2 ReadVectorInt2() =>
            new VectorInt2(ReadInt(), ReadInt());

        public static VectorDouble2 ReadVectorDouble2() =>
            new VectorDouble2(ReadInt(), ReadInt());

        public static string ReplaceX(this string input, string pattern, string replace) =>
            Regex.Replace(input, pattern, replace);

        public static IEnumerable<int> Range(int i, int j) => Enumerable.Range(i, j);
        public static IEnumerable<long> Range(long i, long j) {
            for (long k = i; k < i + j; k++) yield return k;

        }
        public static void Swap<T>(this IList<T> enumerable, int i, int j) {
            var buf = enumerable[i];
            enumerable[i] = enumerable[j];
            enumerable[j] = buf;

        }

        public static bool In<T>(this T t, IEnumerable<T> range) => range.Contains(t);

    }

    static class ParametricExtensions {
        public static Func<S, U> Then<S, T, U>(this Func<S, T> f, Func<T, U> g) => x => g(f(x));
        public static Func<S, U> Compose<S, T, U>(this Func<T, U> f, Func<S, T> g) => x => f(g(x));

        public static void Call<T>(this T t, Action<T> action) => action(t);
        public static void WriteLine<T>(this T t) => Console.WriteLine(t);
        public static void Write<T>(this T t) => Console.Write(t);
        public static T Call<S, T>(this S s, Func<S, T> func) => func(s);

        public static S Call<S>(this S s, Func<S, S> func, int times) => times == 0 ? s : Call(func(s), func, times - 1);

    }

    static class StreamExtensions {

        public static void ReverseRange<T>(this IList<T> enumerable, int i, int j) {
            int half = (j - i) / 2;
            for (int k = 0; k <= half; k++) enumerable.Swap(i + k, j - k);
        }

        public static bool isEmpty<T>(this IEnumerable<T> enumerable) => !enumerable.Any();
        public static void Times(this int n, Action action) {
            for (int i = 0; i < n; i++) action();
        }
        public static void Times(this long n, Action action) {
            for (long i = 0; i < n; i++) action();
        }

        public static IEnumerable<T> Times<T>(this int n, Func<T> func) {
            for (long i = 0; i < n; i++) yield return func();
        }
        public static IEnumerable<T> Times<T>(this long n, Func<T> func) {
            for (long i = 0; i < n; i++) yield return func();
        }

        public static void ForEach<T>(this IEnumerable<T> e, Action<T> action) {
            foreach (var v in e) action(v);
        }
        public static long Prod(this IEnumerable<long> source) => source.Aggregate(Multiply);

        public static IEnumerable<int> FromTo(int a, int b) => Range(a, Max(b - a + 1, 0));
        public static IEnumerable<T> Repeat<T>(T t) {
            while (true) yield return t;
        }
        public static IEnumerable<T> Replicate<T>(int n, T t) => Repeat(t).Take(n);

        public static IEnumerable<T> Cycle<T>(this IEnumerable<T> e) {
            while (true)
                foreach (var v in e) yield return v;
        }

        public static IEnumerable<T> Scan<S, T>(this IEnumerable<S> source, T init, Func<T, S, T> accumulator) {
            var result = init;
            foreach (var item in source) {
                result = accumulator(result, item);
                yield return result;
            }
        }

        public static T[] ToArray<T>(params T[] ns) => ns;
        public static List<T> ToList<T>(params T[] ns) => ns.ToList();

        public static Dictionary<T1, T2> ToDictionary<T1, T2>(this IEnumerable<KeyValuePair<T1, T2>> source) {
            var dict = new Dictionary<T1, T2>();
            source.ForEach(x => dict.Add(x.Key, x.Value));
            return dict;
        }
        public static SortedDictionary<T1, T2> ToSortedDictionary<T1, T2>(this IEnumerable<KeyValuePair<T1, T2>> source) => new SortedDictionary<T1, T2>(source.ToDictionary());
        public static MultiSet<T> ToMultiSet<T>(this IEnumerable<KeyValuePair<T, long>> source) => new MultiSet<T>(source);
        public static SortedMultiSet<T> ToSortedMultiSet<T>(this IEnumerable<KeyValuePair<T, long>> source) => new SortedMultiSet<T>(source);

        public static IEnumerable<T> Scan0<S, T>(this IEnumerable<S> source, T init, Func<T, S, T> accumulator) {
            yield return init;
            foreach (var item in Scan(source, init, accumulator)) yield return item;
        }

        public static IEnumerable<Tuple<T1, T2>> Cartesian<T1, T2>(IEnumerable<T1> s1, IEnumerable<T2> s2) =>
            from v1 in s1 from v2 in s2 select new Tuple<T1, T2>(v1, v2);

        public static IEnumerable<Tuple<T1, T2, T3>> Cartesian<T1, T2, T3>(IEnumerable<T1> s1, IEnumerable<T2> s2, IEnumerable<T3> s3) =>
            from v1 in s1 from v2 in s2 from v3 in s3 select new Tuple<T1, T2, T3>(v1, v2, v3);

        public static IEnumerable<Tuple<T1, T2, T3, T4>> Cartesian<T1, T2, T3, T4>(IEnumerable<T1> s1, IEnumerable<T2> s2, IEnumerable<T3> s3, IEnumerable<T4> s4) =>
            from v1 in s1 from v2 in s2 from v3 in s3 from v4 in s4 select new Tuple<T1, T2, T3, T4>(v1, v2, v3, v4);

        public static IEnumerable<List<T>> Buffer<T>(this IEnumerable<T> source, int length) {
            while (source.Any()) {
                yield return source.Take(length).ToList();
                source = source.Skip(length);
            }
        }

        public static T MaxBy<T>(this IEnumerable<T> source, Func<T, long> selector) {
            T max = source.First();
            foreach (var item in source)
                if (selector(item) > selector(max)) max = item;
            return max;
        }

        public static T MinBy<T>(this IEnumerable<T> source, Func<T, long> selector) {
            T min = source.First();
            foreach (var item in source)
                if (selector(item) < selector(min)) min = item;
            return min;
        }

        public static int IndexOf<T>(this IList<T> source, Func<T, bool> condition) {
            for (int i = 0; i < source.Count; i++)
                if (condition(source[i]))
                    return i;

            return -1;
        }
        public static IEnumerable<List<T>> ChunkBy<T>(this IEnumerable<T> source) => ChunkBy(source, Id, EqualityComparer<T>.Default);
        public static IEnumerable<List<T>> ChunkBy<S, T>(this IEnumerable<T> source, Func<T, S> selector) => ChunkBy(source, selector, EqualityComparer<S>.Default);
        public static IEnumerable<List<T>> ChunkBy<S, T>(this IEnumerable<T> source, Func<T, S> selector, IEqualityComparer<S> comparer) {
            while (source.Any()) {
                var v = selector(source.First());
                var l = source.TakeWhile(x => comparer.Equals(selector(x), v)).ToList();
                yield return l;
                source = source.Skip(l.Count());
            }
        }
    }

    static class Matrix {
        public static List<List<T>> Transpose<T>(this List<List<T>> source) =>
            Range(0, source[0].Count).Select(i => source.Select(x => x[i]).ToList()).ToList();

        public static void WriteMatrix(this IEnumerable<IEnumerable<char>> source) {
            foreach (var line in source) line.Show().WriteLine();
        }

        public static List<List<long>> ReadLongMatrix(int h, int w) => h.Times(() => ReadLong(w)).ToList();

        public static List<List<T>> SelectMatrix<S, T>(this List<List<S>> source, Func<S, T> f) => source.Select(x => x.Select(f).ToList()).ToList();
    }

    static class Cin {
        private static Queue<string> tokens;
        static Cin() {
            string line;
            tokens = new Queue<string>();
            while ((line = Console.ReadLine()) != null) {
                foreach (var token in line.Split(' ')) {
                    tokens.Enqueue(token);
                }
            }
        }

        static public int ReadInt() => int.Parse(tokens.Dequeue());
        static public List<int> ReadInt(long n) {
            var list = new List<int>();
            n.Times(() => list.Add(ReadInt()));
            return list;
        }
        static public long ReadLong() => long.Parse(tokens.Dequeue());
        static public List<long> ReadLong(long n) {
            var list = new List<long>();
            n.Times(() => list.Add(ReadLong()));
            return list;
        }
        static public string ReadString() => tokens.Dequeue();
        static public List<string> ReadString(long n) {
            var list = new List<string>();
            n.Times(() => list.Add(ReadString()));
            return list;
        }

        static public void SayYesNo(this bool b) => (b ? "Yes" : "No").WriteLine();
        static public void SayYESNO(this bool b) => (b ? "YES" : "NO").WriteLine();

    }

    public struct VectorInt2 {
        public int X { get; set; }
        public int Y { get; set; }

        public VectorInt2(int x, int y) {
            X = x;
            Y = y;
        }

        static public VectorInt2 operator +(VectorInt2 v1, VectorInt2 v2) =>
            new VectorInt2(v1.X + v2.X, v1.Y + v2.Y);
        static public VectorInt2 operator -(VectorInt2 v1, VectorInt2 v2) =>
            new VectorInt2(v1.X - v2.X, v1.Y - v2.Y);
        static public VectorInt2 operator *(VectorInt2 v1, VectorInt2 v2) =>
            new VectorInt2(v1.X * v2.X, v1.Y * v2.Y);
        static public VectorInt2 operator *(VectorInt2 v1, int i) =>
            new VectorInt2(v1.X * i, v1.Y * i);
        static public VectorInt2 operator *(int i, VectorInt2 v2) =>
            new VectorInt2(i * v2.X, i * v2.Y);
        static public VectorInt2 operator /(VectorInt2 v1, int i) =>
            new VectorInt2(v1.X / i, v1.Y / i);

        public override string ToString() => $"({X}, {Y})";
    }

    struct VectorDouble2 {
        public double X { get; set; }
        public double Y { get; set; }

        public VectorDouble2(double x, double y) {
            X = x;
            Y = y;
        }

        static public VectorDouble2 operator +(VectorDouble2 v1, VectorDouble2 v2) =>
            new VectorDouble2(v1.X + v2.X, v1.Y + v2.Y);
        static public VectorDouble2 operator -(VectorDouble2 v1, VectorDouble2 v2) =>
            new VectorDouble2(v1.X - v2.X, v1.Y - v2.Y);
        static public VectorDouble2 operator *(VectorDouble2 v1, VectorDouble2 v2) =>
            new VectorDouble2(v1.X * v2.X, v1.Y * v2.Y);
        static public VectorDouble2 operator *(VectorDouble2 v1, int i) =>
            new VectorDouble2(v1.X * i, v1.Y * i);
        static public VectorDouble2 operator *(int i, VectorDouble2 v2) =>
            new VectorDouble2(i * v2.X, i * v2.Y);
        static public VectorDouble2 operator /(VectorDouble2 v1, double i) =>
            new VectorDouble2(v1.X / i, v1.Y / i);

        public override string ToString() => $"({X}, {Y})";

        public VectorDouble2 Turn(double radian) => new VectorDouble2(X * Cos(radian) - Y * Sin(radian), X * Sin(radian) + Y * Cos(radian));
    }

    public struct LongRange {
        public long Left;
        public long Right;

        public LongRange(long left, long right) {
            Left = left;
            Right = right;
        }
    }

    public struct LongFromToVector {
        public VectorInt2 Start;
        public VectorInt2 End;
        public LongFromToVector(VectorInt2 start, VectorInt2 end) {
            Start = start;
            End = end;
        }
    }
}

namespace AtCoder {
    using System.Collections.Generic;
    using System.Collections;
    using System.Linq;
    using System;

    class MultiSet<T> : ICollection<T> {
        // TODO: 順番がグチャグチャなので整理する
        Dictionary<T, long> occurrence;

        public MultiSet() { occurrence = new Dictionary<T, long>(); }
        public MultiSet(MultiSet<T> source) : this(source.occurrence) {}
        public MultiSet(Dictionary<T, long> source) {
            occurrence = new Dictionary<T, long>(source);
            Validate();
        }

        public MultiSet(IEnumerable<T> source) {
            occurrence = new Dictionary<T, long>();
            AddRange(source);
        }

        public MultiSet(IEnumerable<KeyValuePair<T, long>> source) {
            occurrence = new Dictionary<T, long>();
            foreach (var item in source) this [item.Key] += item.Value;
        }

        public void Add(T data) {
            if (!occurrence.ContainsKey(data)) {
                occurrence[data] = 1;
                return;
            }
            occurrence[data]++;
        }

        public void Clear() => occurrence.Clear();

        public void Add(T key, int value) {
            if (value <= 0) throw new ArgumentException();
            if (!occurrence.ContainsKey(key)) {
                occurrence[key] = value;
                return;
            }
            occurrence[key] += value;
        }

        public void Add(KeyValuePair<T, int> pair) => Add(pair.Key, pair.Value);

        public void AddRange(IEnumerable<T> source) =>
        source.ToList().ForEach(Add);

        public bool Remove(T data) {
            if (!occurrence.ContainsKey(data)) return false;
            occurrence[data]--;
            if (occurrence[data] == 0) occurrence.Remove(data);
            return true;
        }

        // IDictionary
        public long this [T key] {
            get { return occurrence.ContainsKey(key) ? occurrence[key] : 0; }
            set {
                if (value <= 0) throw new ArgumentException();
                occurrence[key] = value;
            }
        }

        public ICollection<T> Keys {
            get { return occurrence.Keys; }
        }
        public ICollection<long> Values {
            get { return occurrence.Values; }
        }

        public bool ContainsKey(T key) => occurrence.ContainsKey(key);
        public bool Contains(T key) => ContainsKey(key);
        public bool TryGetValue(T key, out long value) {
            value = this [key];
            return true;
        }
        // /IDictionary

        public long LongCount() => occurrence.Values.Sum();
        public int Count => (int) occurrence.Values.Sum();
        public bool IsReadOnly => false;
        public void CopyTo(T[] array, int index) {
            foreach (var pair in occurrence) {
                for (int i = 0; i < pair.Value; i++) {
                    array[index] = pair.Key;
                    index++;
                }
            }
        }

        public IEnumerator<T> GetEnumerator() {
            foreach (var pair in occurrence)
                for (int i = 0; i < pair.Value; i++) yield return pair.Key;

        }

        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();

        public void Validate() {
            if (occurrence.Values.Any(x => x <= 0)) throw new ArgumentException();
        }
    }

    class SortedMultiSet<T> : ICollection<T> {
        // TODO: 順番がグチャグチャなので整理する
        SortedDictionary<T, long> occurrence;

        public SortedMultiSet() { occurrence = new SortedDictionary<T, long>(); }
        public SortedMultiSet(SortedMultiSet<T> source) : this(source.occurrence) {}
        public SortedMultiSet(SortedDictionary<T, long> source) {
            occurrence = new SortedDictionary<T, long>(source);
            Validate();
        }

        public SortedMultiSet(IEnumerable<T> source) {
            occurrence = new SortedDictionary<T, long>();
            AddRange(source);
        }

        public SortedMultiSet(IEnumerable<KeyValuePair<T, long>> source) {
            occurrence = new SortedDictionary<T, long>();
            foreach (var item in source) this [item.Key] += item.Value;
        }

        public void Add(T data) {
            if (!occurrence.ContainsKey(data)) {
                occurrence[data] = 1;
                return;
            }
            occurrence[data]++;
        }

        public void Clear() => occurrence.Clear();

        public void Add(T key, int value) {
            if (value <= 0) throw new ArgumentException();
            if (!occurrence.ContainsKey(key)) {
                occurrence[key] = value;
                return;
            }
            occurrence[key] += value;
        }

        public long LongCount() => occurrence.Values.Sum();
        public int Count => (int) occurrence.Values.Sum();

        public void Add(KeyValuePair<T, int> pair) => Add(pair.Key, pair.Value);

        public void AddRange(IEnumerable<T> source) =>
        source.ToList().ForEach(Add);

        public bool Remove(T data) {
            if (!occurrence.ContainsKey(data)) return false;
            occurrence[data]--;
            if (occurrence[data] == 0) occurrence.Remove(data);
            return true;
        }

        // IDictionary
        public long this [T key] {
            get {
                return occurrence.ContainsKey(key) ? occurrence[key] : 0;
            }
            set {
                if (value <= 0) throw new ArgumentException();
                occurrence[key] = value;
            }
        }

        public ICollection<T> Keys => occurrence.Keys;
        public ICollection<long> Values => occurrence.Values;

        public bool ContainsKey(T key) => occurrence.ContainsKey(key);
        public bool Contains(T key) => ContainsKey(key);
        public bool TryGetValue(T key, out long value) {
            value = this [key];
            return true;
        }
        // /IDictionary

        public bool IsReadOnly => false;
        public void CopyTo(T[] array, int index) {
            foreach (var pair in occurrence) {
                for (int i = 0; i < pair.Value; i++) {
                    array[index] = pair.Key;
                    index++;
                }
            }
        }

        public IEnumerator<T> GetEnumerator() {
            foreach (var pair in occurrence)
                for (int i = 0; i < pair.Value; i++) yield return pair.Key;

        }

        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();

        public void Validate() {
            if (occurrence.Values.Any(x => x <= 0)) throw new ArgumentException();
        }
    }
}