Clojurians Log v2

I need to compare some messy relational data and I’m looking for a data structure that is like a set, but compares elements according to a function other than = , such that

(conj
  (cool-data-structure :relevant-key #{{:relevant-key :A :other-key :value2} ... lots more like this})
  {:relevant-key :A, :other-key :DIFFERENT-VALUE}
  {:relevant-key :B, :other-key :whatever}
   ... again many more records
)

would produce

#cool-data-structure{{:relevant-key :A, :other-key :DIFFERENT_VALUE}
                     {:relevant-key :B, :other-key :whatever}
...}

I think this would be pretty easy to write but wonder if there is prior art