MODULE ::= EXPR*
EXPR ::=
| TERM
| ASSERT
ASSERT ::= (assert_eq|assert_ne) tuple
TERM ::= FACTOR ((add|sub) FACTOR)*
FACTOR ::= USUB ((mul|div|mod) USUB)*
USUB ::= sub* POW
POW ::= LITERAL (pow LITERAL)?
LITERAL ::= 
| BUILTIN 
| TUPLE
| opar EXPR cpar
TUPLE ::=
| opar EXPR+ cpar
BUILTIN ::= num