[Asdf-devel] Why does asdf use ``operate''?

Why does ASDF use the (operate operation-name arguments ...) convention instead of the more usual function call (function-name arguments ...) Even with `operate', eventually a function will be called. Also that one can define top level functions that call operate (defun foo (...) (operate 'foo-operation ...)) What does the extra layer of operate allow that generic functions and methods do not? Thank you, Mirko