PROG ::= INSTR*
INSTR ::=
| DIR
| EXPR semicolon

DIR ::= hash ident EXPR

EXPR ::=
| ADDSUB

ADDSUB ::= MULDIVMOD ((add|sub) MULDIVMOD)*
MULDIVMOD ::= LITERAL ((mul|div|mod) LITERAL)*

LITERAL ::=
| ident
| int