Add find snippets

snippets/find_last_index.md

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+---
+title: find_last_index
+tags: list,beginner
+---
+
+Returns the index of the last element in the provided list that satisfies the provided testing function.
+
+Use list comprehension, `enumerate()` and `next()` to return the index of the last element in `lst` for which `fn` returns `True`.
+
+```py
+def find_last_index(lst, fn):
+  return len(lst) - 1 - next(i for i, x in enumerate(lst[::-1]) if fn(x))
+```
+
+```py
+find_last_index([1, 2, 3, 4], lambda n: n % 2 == 1) # 2
+```