How can I list the items deleted from DeleteDuplicatesBy
?
For example:
DeleteDuplicatesBy[{{a, b}, {f, c}, {d, b}, {e, c}, {a, c}}, Part[#, 2] &]
gives
{{a, b}, {f, c}}
However, is there any simple way to get the list {{d, b}, {e, c}, {a, c}}
of deleted pairs?