Apply a unary operation to the value corresponding to key. If there isn't such a value, return a V.init wrapped inside a Nullable.
the key corresponding to a value
a nullable value corresponding to the result or V.init.
Complexity: O(n), where n is the number of elements in the corresponding key bucket.
See Implementation
Apply a unary operation to the value corresponding to key. If there isn't such a value, return a V.init wrapped inside a Nullable.