expression -> equality ;
equality   -> comparison ( ( "!=" | "==" ) comparison )* ;
comparison -> term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
term       -> factor ( ( "-" | "+" ) factor )* ;
factor     -> exponent ( ( "/" | "*" | "/." | "/_" | "%" | "/%" ) exponent )* ;
exponent   -> unary ( ( "**" ) unary )* ;
unary      -> ( "!" | "-" ) unary
           | primary ;
primary    -> NUMBER | STRING | "true" | "false" | "nil"
           | "(" expression ")" ;