Arguably API design is the most creative aspect of programming. The space of possible interfaces is infinite, whereas function implementations are heavily const

Arguably API design is the most creative aspect of programming. The space of possible interfaces is infinite, whereas function implementations are heavily constrained by the interface and often there is only a handful of ways to write an implementation for a given interface. Two equally good programmers may come up with radically different interfaces. Whereas if given an interface and instructed to implement it, they are likely to converge to a similar implementation.