Eventually, when parsing problems are fixed, we should allow tuples as
builtin and as extending normal expressions.  We want to write f(1,2,3)
meaning that the tuple (1,2,3) is passed to f, while we can write
either f 1 or f(1) for one argument invocations.  For now, use
constructors for tuples, for example f Triple(1,2,3), etc.
