Exactly. For this to work there needs to be the constraint that there's a one-to-one mapping in each direction. The Bimap should have the uniqueness promise that "Set (k, v)" gives. Yet you should be able to search on either tuple value.
Or... have the possibility of returning a list of values. Arguably there are two possible implementations, one that enforces one-to-one mapping, and one which allows multiple values, in either direction. "But how can you change a value if there are non-unique keys?". Well, you dont change a value, you change a list of values ;-) So, let's say our bimap is: 1,1 1,2 5,2 5,3 then: bimap_getvalue ourbimap 1 gives [1,2] bimap_getkey ourbimap 2 gives [1,5] Executing bimap_setkey ourbimap 2 [1,4] changes the bimap to: 1,1 1,2 4,2 5,3