ROOT ::= EXPR*
EXPR ::= 
| OR
| ASSERT
| VAR_DECL
| CONST_DECL
| ASSIGN
| BLOCK
| IF
| return EXPR
IF ::= if EXPR BLOCK (else (BLOCK | IF))?
BLOCK ::= begin EXPR* end
ASSIGN ::= ident assign EXPR
VAR_DECL ::= var ident assign EXPR
CONST_DECL ::= const ident assign EXPR
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
| ident
| FUN
| CALL
CALL ::= ident opar ARGS cpar
ARGS ::= (EXPR (comma EXPR)*)?
FUN ::= fun opar PARAMS cpar EXPR* end
PARAMS ::= (EXPR (comma EXPR)*)?
BUILTIN ::=
| int
| bool
| float
| string