The dragon's curve is a self-similar fractal which can be obtained by a recursive method. 

Starting with the string `D0 = 'Fa'`, at each step simultaneously perform the following operations:

```
replace 'a' with: 'aRbFR'
replace 'b' with: 'LFaLb'
```

For example (spaces added for more visibility) : 

```
1st iteration: Fa -> F aRbF R
2nd iteration: FaRbFR -> F aRbFR R LFaLb FR
```

After `n` iteration, remove `'a'` and `'b'`. You will have a string with `'R'`,`'L'`, and `'F'`. This is a set of instruction. Starting at the origin of a grid looking in the `(0,1)` direction, `'F'` means a step forward, `'L'` and `'R'` mean respectively turn left and right. After executing all instructions, the trajectory will give a beautifull self-replicating pattern called 'Dragon Curve'

The goal of this kata is to code a function wich takes one parameter `n`, the number of iterations needed and return the string of instruction as defined above. For example: 

```
n=0, should return: 'F'
n=1, should return: 'FRFR'
n=2, should return: 'FRFRRLFLFR'
```

`n` should be a number and non-negative integer. All other case should return the empty string: `''`.

```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 = "def Dragon(n):\n    if not isinstance(n, int) or n < 0:\n        return ''\n        \n    value = 'Fa'\n    \n    for i in range(n):\n        value = value.replace('a', 'aRcFR')\n        value = value.replace('b', 'LFaLb')\n        value = value.replace('c', 'b')\n        \n    return value.replace('a', '').replace('b', '')"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'Dragon'
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: 5576