Nested scopes could never have worked. My fault I wrote such a buggy code,
Pavel's fault that he's never tested shadowing of declarations in the filters.
cf_define_symbol() has been modified to check the scope of the symbol it's
given and it if it's an already defined symbol, but in a different scope,
a copy is created in the current scope and redefined to the new meaning,
the consequence being that it cf_define_symbol() now returns the new symbol
you need to use when assigning aux and aux2.