The problem is how to statically ensure that you don’t run into the case of a missing implementation for some combination at runtime, in particular in languages with separate compilation. If you have a library A that defines a basic (T, O) set, and two libraries B and C that aren’t aware of each other but both use A, where B adds a new type and C adds a new operation, and finally you have a program D that uses both B and C, how do you statically detect or prevent the case where D tries to use the new type from B with the new operation from C, but no one has provided an implementation for that combination; while also supporting the case of the required implementation being present.