lookup package:unordered-containers
Return the value to which the specified key is mapped, or
Nothing if this map contains no mapping for the key.
lookup' is a version of lookup that takes the hash separately. It is
used to implement alterF.
Return the value to which the specified key is mapped, or
the default value if this map contains no mapping for the key.
DEPRECATED: lookupDefault is deprecated as of version 0.2.11, replaced
by
findWithDefault.