Simplified the example arithmetics grammar.

examples/arithmetics.peg

@@ -1,21 +1,15 @@
-start          : _ expression { return $2; }
+start          : _ additive { return $2; }
 
-expression     : additive
-
-additive       : multiplicative (plus / minus) additive {
-                   if ($2 === "+") { return $1 + $3; }
-                   if ($2 === "-") { return $1 - $3; }
-                 }
+additive       : multiplicative plus  additive { return $1 + $3; }
+               / multiplicative minus additive { return $1 - $3; }
                / multiplicative
 
-multiplicative : primary (times / divide) multiplicative {
-                   if ($2 === "*") { return $1 * $3; }
-                   if ($2 === "/") { return $1 / $3; }
-                 }
+multiplicative : primary times  multiplicative { return $1 * $3; }
+               / primary divide multiplicative { return $1 / $3; }
                / primary
 
 primary        : integer
-               / lparen expression rparen { return $2 }
+               / lparen additive rparen { return $2 }
 
 integer        "integer"
                : [0-9]+ _ { return parseInt($1.join("")); }