We need to either
a) Make the grammar more regular with respect to control structures
(maybe a standard block syntax, like C's {}s or Lisp's ()s?),
b) Require that control structures end with some standard
terminator, or
c) Forbid extension control structures.
d) Forbid extension control structures unless script also includes a
"require <extension>" prior to their use.
I'm partial to (d). :-)