This kata generalizes [Twice Linear](https://www.codewars.com/kata/5672682212c8ecf83e000050). You may want to attempt that kata first.

## Sequence

Consider an integer sequence `U(m)` defined as:

1. `m` is a given non-empty set of positive integers.
2. `U(m)[0] = 1`, the first number is always 1.
3. For each `x` in `U(m)`, and each `y` in `m`, `x * y + 1` must also be in `U(m)`.
4. No other numbers are in `U(m)`.
5. `U(m)` is sorted, with no duplicates.

### Sequence Examples

#### `U(2, 3) = [1, 3, 4, 7, 9, 10, 13, 15, 19, 21, 22, 27, ...]`

1 produces 3 and 4, since `1 * 2 + 1 = 3`, and `1 * 3 + 1 = 4`.

3 produces 7 and 10, since `3 * 2 + 1 = 7`, and `3 * 3 + 1 = 10`.

#### `U(5, 7, 8) = [1, 6, 8, 9, 31, 41, 43, 46, 49, 57, 64, 65, 73, 156, 206, ...]`

1 produces 6, 8, and 9.

6 produces 31, 43, and 49.

## Task:

Implement `n_linear` or `nLinear`: given a set of postive integers `m`, and an index `n`, find `U(m)[n]`, the `n`th value in the `U(m)` sequence.

### Tips

* Tests use large n values. Slow algorithms may time-out.
* Tests use large values in the m set. Algorithms which multiply further than neccessary may overflow.
* Linear run time and memory usage is possible.
* How can you build the sequence iteratively, without growing extra data structures?

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'from heapq import heappush, heappop\n\ndef n_linear(m, n):\n    h = [1]\n    x = 0\n    for _ in range(n+1):\n        x = heappop(h)\n        while h and h[0] == x:\n            heappop(h)\n        for y in m:\n            a = y * x + 1\n            heappush(h, a)\n    return x'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'n_linear'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5492