Task: Stream merge
C++: 
#include <iostream>
#include <vector>

template <typename C, typename A>
void merge2(const C& c1, const C& c2, const A& action) {
    auto i1 = std::cbegin(c1);
    auto i2 = std::cbegin(c2);

    while (i1 != std::cend(c1) && i2 != std::cend(c2)) {
        if (*i1 <= *i2) {
            action(*i1);
            i1 = std::next(i1);
        } else {
            action(*i2);
            i2 = std::next(i2);
        }
    }
    while (i1 != std::cend(c1)) {
        action(*i1);
        i1 = std::next(i1);
    }
    while (i2 != std::cend(c2)) {
        action(*i2);
        i2 = std::next(i2);
    }
}

template <typename A, typename C>
void mergeN(const A& action, std::initializer_list<C> all) {
    using I = typename C::const_iterator;
    using R = std::pair<I, I>;

    std::vector<R> vit;
    for (auto& c : all) {
        auto p = std::make_pair(std::cbegin(c), std::cend(c));
        vit.push_back(p);
    }

    bool done;
    R* least;
    do {
        done = true;

        auto it = vit.begin();
        auto end = vit.end();
        least = nullptr;

        
        while (it != end && it->first == it->second) {
            it++;
        }
        if (it != end) {
            least = &(*it);
        }
        while (it != end) {
            
            while (it != end && it->first == it->second) {
                it++;
            }
            if (least != nullptr && it != end
                && it->first != it->second
                && *(it->first) < *(least->first)) {
                
                least = &(*it);
            }
            if (it != end) {
                it++;
            }
        }
        if (least != nullptr && least->first != least->second) {
            done = false;
            action(*(least->first));
            least->first = std::next(least->first);
        }
    } while (!done);
}

void display(int num) {
    std::cout << num << ' ';
}

int main() {
    std::vector<int> v1{ 0, 3, 6 };
    std::vector<int> v2{ 1, 4, 7 };
    std::vector<int> v3{ 2, 5, 8 };

    merge2(v2, v1, display);
    std::cout << '\n';

    mergeN(display, { v1 });
    std::cout << '\n';

    mergeN(display, { v3, v2, v1 });
    std::cout << '\n';
}

Go: package main

import (
    "container/heap"
    "fmt"
    "io"
    "log"
    "os"
    "strings"
)

var s1 = "3 14 15"
var s2 = "2 17 18"
var s3 = ""
var s4 = "2 3 5 7"

func main() {
    fmt.Print("merge2: ")
    merge2(
        os.Stdout,
        strings.NewReader(s1),
        strings.NewReader(s2))
    fmt.Println()

    fmt.Print("mergeN: ")
    mergeN(
        os.Stdout,
        strings.NewReader(s1),
        strings.NewReader(s2),
        strings.NewReader(s3),
        strings.NewReader(s4))
    fmt.Println()
}

func r1(r io.Reader) (v int, ok bool) {
    switch _, err := fmt.Fscan(r, &v); {
    case err == nil:
        return v, true
    case err != io.EOF:
        log.Fatal(err)
    }
    return
}

func merge2(m io.Writer, s1, s2 io.Reader) {
    v1, d1 := r1(s1)
    v2, d2 := r1(s2)
    var v int
    for d1 || d2 {
        if !d2 || d1 && v1 < v2 {
            v = v1
            v1, d1 = r1(s1)
        } else {
            v = v2
            v2, d2 = r1(s2)
        }
        fmt.Fprint(m, v, " ")
    }
}

type sv struct {
    s io.Reader
    v int
}

type sh []sv

func (s sh) Len() int            { return len(s) }
func (s sh) Less(i, j int) bool  { return s[i].v < s[j].v }
func (s sh) Swap(i, j int)       { s[i], s[j] = s[j], s[i] }
func (p *sh) Push(x interface{}) { *p = append(*p, x.(sv)) }
func (p *sh) Pop() interface{} {
    s := *p
    last := len(s) - 1
    v := s[last]
    *p = s[:last]
    return v
}

func mergeN(m io.Writer, s ...io.Reader) {
    var h sh
    for _, s := range s {
        if v, d := r1(s); d {
            h = append(h, sv{s, v})
        }
    }
    heap.Init(&h)
    for len(h) > 0 {
        p := heap.Pop(&h).(sv)
        fmt.Fprint(m, p.v, " ")
        if v, d := r1(p.s); d {
            heap.Push(&h, sv{p.s, v})
        }
    }
}