Like Set Tools in 'Base' Package but Keeps Duplicate Elements
Description
The 'base' tools union() intersect(), etc., follow the algebraic definition
that each element of a set must be unique.
Since it's often helpful to compare all elements of two vectors,
this toolset treats every element as unique for counting purposes.
For ease of use, all functions in vecsets have an argument
'multiple' which, when set to FALSE,
reverts them to the base::sets (alias for all the items) tools functionality.