Add union and union_by snippets

snippets/union_by.md

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+---
+title: union_by
+tags: list,function,intermediate
+---
+
+Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.
+
+Create a `set` by applying `fn` to each element in `a`, then use list comprehension in combination with `fn` on `b` to only keep values not contained in the previously created set, `_a`.
+Finally, create a `set` from the previous result and `a` and transform it into a `list`
+
+```py
+def union_by(a,b,fn):
+  _a = set(map(fn, a))
+  return list(set(a + [item for item in b if fn(item) not in _a]))
+```
+
+```py
+from math import floor
+union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]
+```