I think the point was that you can't tell if `foo bar` is equivalent to `bar(foo())` or `bar(foo(value_from_stack))` or `bar(foo(val1, val2))` or `foo(); bar()`

I think the point was that you can't tell if `foo bar` is equivalent to `bar(foo())` or `bar(foo(value_from_stack))` or `bar(foo(val1, val2))` or `foo(); bar()` or what unless you know the stack effect of the words.