<expr> ::= <expr>0 "<" <expr> | <expr>0 "<=" <expr> 
    | <expr>0 "==" <expr> | <expr>0 ">" <expr> | <expr>0 ">=" <expr> | <expr>0 
<expr>0 = <expr>1 "+" <expr> | <expr>1 "-" <expr> | <expr>1 
<expr>1 ::= <atom> "*" <expr>1 | <atom> "/" <expr>1 | <atom> 
<atom> ::= "(" <expr> ")" | <NUMBER> 
