start          : _ expression { return $2; }

expression     : additive

additive       : multiplicative (plus / minus) additive {
                   if ($2 === "+") { return $1 + $3; }
                   if ($2 === "-") { return $1 - $3; }
                 }
               / multiplicative

multiplicative : primary (times / divide) multiplicative {
                   if ($2 === "*") { return $1 * $3; }
                   if ($2 === "/") { return $1 / $3; }
                 }
               / primary

primary        : integer
               / lparen expression rparen { return $2 }

integer        "integer"
               : [0-9]+ _ { return parseInt($1.join("")); }

plus           : "+" _ { return $1; }
minus          : "-" _ { return $1; }
times          : "*" _ { return $1; }
divide         : "/" _ { return $1; }
lparen         : "(" _
rparen         : ")" _

_              "whitespace"
               : [ \t\n\r]*