ROOT ::= EXPR*
EXPR ::= 
| OR
| ASSERT
ASSERT ::= assert EXPR eq EXPR
OR ::= AND (or AND)*
AND ::= TERM (and TERM)*
TERM ::= FACTOR ((add|sub) FACTOR)*
FACTOR ::= USUB ((mul|div|mod) USUB)*
USUB ::= sub* NOT 
NOT ::= not* POW
POW ::= LITERAL (pow LITERAL)?
LITERAL ::=
| BUILTIN 
| opar EXPR cpar
BUILTIN ::=
| int
| bool