PROG ::= (INSTR (EOI INSTR)*)?
INSTR ::= EXPR
EXPR ::= TERM

TERM ::= FACTOR ((add | sub) FACTOR)*
FACTOR ::= UNOP ((mul | div | mod) UNOP)*

UNOP ::= (add | sub)? POW
POW ::= GROUP (pow GROUP)?
GROUP ::= BASE | opar EXPR cpar
BASE ::= int