partition
Given a list and a key-extraction function, produce a list of non-empty lists of elements that have the same key. Note that it only looks for runs of a key, and treats reoccurrences of some key after a gap to be a new key.
Receiver
The list of elements.
Return
A list (empty iff the receiver is empty) of non-empty lists, each producing the same key. Note that flattening this list produces the original list.
Parameters
grouping
The key extraction lambda.