method e set_error msg
arg_rw Expression e ; arg Str msg
Notify that the expression is not valid.When the expression does not look as expected by your meta programming function, you should return without calling set_error method in most cases because another meta function could be satisfyed with it. If you set the error, no other function get a chance to compile the expression, so you bring polymorphism down.
method e unset_error
arg_rw Expression e
Clears the error condition on the expression.