Add symmetric_difference and symmetric_difference_by snippets

snippets/symmetric_difference.md

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+---
+title: symmetric_difference
+tags: list,beginner
+---
+
+Returns the symmetric difference between two iterables, without filtering out duplicate values.
+
+Create a `set` from each list, then use list comprehension on each one to only keep values not contained in the previously created set of the other.
+
+```py
+def symmetric_difference(a, b):
+  _a, _b = set(a), set(b)
+  return [item for item in a if item not in _b] + [item for item in b if item not in _a]
+```
+
+```py
+symmetric_difference([1, 2, 3], [1, 2, 4]) # [3, 4]
+```

snippets/symmetric_difference_by.md

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+---
+title: symmetric_difference_by
+tags: list,function,intermediate
+---
+
+Returns the symmetric difference between two lists, after applying the provided function to each list element of both.
+
+Create a `set` by applying `fn` to each element in every list, then use list comprehension in combination with `fn` on each one to only keep values not contained in the previously created set of the other.
+
+```py
+def symmetric_difference_by(a, b, fn):
+  _a, _b = set(map(fn, a)), set(map(fn, b))
+  return [item for item in a if fn(item) not in _b] + [item for item in b if fn(item) not in _a]
+```
+
+```py
+from math import floor
+symmetric_difference_by([2.1, 1.2], [2.3, 3.4],floor) # [1.2, 3.4]
+```