syntax confusion with function/non-function objects (PR#694)

In general you can have any expression evaluating to the function
to be called. If you do f(x)(3), f(x) may or may not be a function
and you can only detect whether it is one at runtime.

I agree--I'd call this a type error in evaluation, not a syntax error.

One thing to keep in mind is that R expressions can be used for things
other that evaluation, for example in representing aspects of models.
Someone might want at some point to attach meaning to something like

	y ~ 2(x)+z

That would be impossible (or at least very dificult) if the expression
were made syntactically invalid.

It would be possible to have a semantic checking tool for R that walks
over the code and warns when it sees such an expression, but it should
not be considered an error since it could be legitimate.