Your task is to Combine two Strings. But consider the rule...

By the way you don't have to check errors or incorrect input values, everything is ok without bad tricks, only two input strings and as result one output string;-)...

And here's the rule:  
Input Strings `a` and `b`: For every character in string `a` swap the casing of every occurrence of the same character in string `b`. Then do the same casing swap with the inputs reversed. Return a single string consisting of the changed version of `a` followed by the changed version of `b`. A char of `a` is in `b` regardless if it's in upper or lower case - see the testcases too.  
I think that's all;-)...

Some easy examples:

````
Input: "abc" and "cde"      => Output: "abCCde" 
Input: "ab" and "aba"       => Output: "aBABA"
Input: "abab" and "bababa"  => Output: "ABABbababa"
````

Once again for the last example - description from `KenKamau`, see discourse;-):

a) swap the case of characters in string `b` for every occurrence of that character in string `a`  
char `'a'` occurs twice in string `a`, so we swap all `'a'` in string `b` twice. This means we start with `"bababa"` then `"bAbAbA"` => `"bababa"`  
char `'b'` occurs twice in string `a` and so string `b` moves as follows: start with `"bababa"` then `"BaBaBa"` => `"bababa"`

b) then, swap the case of characters in string `a` for every occurrence in string `b`  
char `'a'` occurs `3` times in string `b`. So string `a` swaps cases as follows: start with `"abab"` then => `"AbAb"` => `"abab"` => `"AbAb"`  
char `'b'` occurs `3` times in string `b`. So string `a` swaps as follow: start with `"AbAb"` then => `"ABAB"` => `"AbAb"` => `"ABAB"`.

c) merge new strings `a` and `b`  
return `"ABABbababa"`

There are some static tests at the beginning and many random tests if you submit your solution.

Hope you have fun:-)!

```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 work_on_strings(a, b):\n    new_a = [letter if b.lower().count(letter.lower()) % 2 == 0 else letter.swapcase() for letter in a]\n    new_b = [letter if a.lower().count(letter.lower()) % 2 == 0 else letter.swapcase() for letter in b]\n    return ''.join(new_a) + ''.join(new_b)"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'work_on_strings'
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: 6090