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MERGE-FN M1 M2procedure (MERGE-FN X Y) gives the value of a key that is in both maps, where X is its value in M1 and Y its value in M2. Keys in only one map keep their value.
(MERGE-FN X Y) gives the value of a key that is in both maps, where X is its value in M1 and Y its value in M2. Keys in only one map keep their value.