Leonardo Fibonacci in a rare portrait of his younger days

I assume you are all familiar with the famous Fibonacci sequence, having to get each number as the sum of the previous two (and typically starting with either `[0,1]` or `[1,1]` as the first numbers).

While there are plenty of variation on it ([including](https://www.codewars.com/kata/tribonacci-sequence) [a few](https://www.codewars.com/kata/fibonacci-tribonacci-and-friends) [I wrote](https://www.codewars.com/kata/triple-shiftian-numbers/)), usually the catch is all the same: get a starting (signature) list of numbers, then proceed creating more with the given rules.

What if you were to get to get two parameters, one with the signature (starting value) and the other with the number you need to sum at each iteration to obtain the next one?

And there you have it, getting 3 parameters:

* a signature of length `length`
* a second parameter is a list/array of indexes of the last `length` elements you need to use to obtain the next item in the sequence (consider you can end up not using a few or summing the same number multiple times)' in other words, if you get a signature of length `5` and `[1,4,2]` as indexes, at each iteration you generate the next number by summing the 2nd, 5th and 3rd element (the ones with indexes `[1,4,2]`) of the last 5 numbers
* a third and final parameter is of course which sequence element you need to return (starting from zero, I don't want to bother you with adding/removing 1s or just coping with the fact that after years on CodeWars we all count as computers do):

```python
custom_fib([1,1],[0,1],2) == 2 #classical fibonacci!
custom_fib([1,1],[0,1],3) == 3 #classical fibonacci!
custom_fib([1,1],[0,1],4) == 5 #classical fibonacci!
custom_fib([3,5,2],[0,1,2],4) == 17 #similar to my Tribonacci
custom_fib([7,3,4,1],[1,1],6) == 2 #can you figure out how it worked ;)?
```

```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 = 'custom_fib=lambda s,x,n,l=None: s[n] if len(s)>n else custom_fib((lambda p: s+[sum(p[i] for i in x)])(s[-(l or len(s)):]),x,n,l or len(s))'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'custom_fib'
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: 6176