ROOT ::= EXPR*
EXPR ::= 
| OR
| ASSERT
ASSERT ::= assert EXPR eq EXPR
OR ::= AND (or AND)*
AND ::= EQ (and EQ)*
EQ ::= CMP ((equal|not_equal) CMP)?
CMP ::= TERM ((lt|gt|le|ge) 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
| float
| string