Exp0 - BNF

<program> ::= <statement>
        | <program> <statement>

<statement> ::= "p" <expression> ";"
        | "s" <leftvar> ";"

<expression> ::= "+" <expression> <expression>
        | "-" <expression> <expression>
        | "(" <expression> <expression>
        | <rightvar>
        | <number>

<leftvar> ::= "x" | "y" | "z"

<rightvar> ::= "x" | "y" | "z"

<number> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' |'9'