> We can't do that, however, because Prolog doesn't support unification of functors. We could patch that if we started referring to function application with more elaborate syntax by parsing "sin(exp(x))" as application(sin, application(exp, x)) so that we could unify on sin, but this quickly obscures the syntax tree and removes the delightfully direct nature of DCGs.

Would the =../2 predicate help? It allows for you to unify functors without wrapping them in something like your application(exp, x) example. Not sure how it would interact with DCGs, though - never took more than a superficial glance at them.