Simplified the example arithmetics grammar once more + addded comment.

examples/arithmetics.pegjs

@@ -1,25 +1,19 @@
-start          : _ additive { return $2; }
+/*
+ * Classic example grammar, which recognizes simple arithmetic expressions like
+ * "2*(3+4)". The parser generated from this grammar then computes their value.
+ */
 
-additive       : multiplicative plus  additive { return $1 + $3; }
+start             : additive
-               / multiplicative minus additive { return $1 - $3; }
-               / multiplicative
 
-multiplicative : primary times  multiplicative { return $1 * $3; }
+additive          : multiplicative "+" additive { return $1 + $3; }
-               / primary divide multiplicative { return $1 / $3; }
+                  / multiplicative "-" additive { return $1 - $3; }
-               / primary
+                  / multiplicative
 
-primary        : integer
+multiplicative    : primary "*" multiplicative { return $1 * $3; }
-               / lparen additive rparen { return $2 }
+                  / primary "/" multiplicative { return $1 / $3; }
+                  / primary
 
-integer        "integer"
+primary           : integer
-               : [0-9]+ _ { return parseInt($1.join("")); }
+                  / "(" additive ")" { return $2; }
 
-plus           : "+" _ { return $1; }
+integer "integer" : [0-9]+ { return parseInt($1.join("")); }
-minus          : "-" _ { return $1; }
-times          : "*" _ { return $1; }
-divide         : "/" _ { return $1; }
-lparen         : "(" _
-rparen         : ")" _
-
-_              "whitespace"
-               : [ \t\n\r]*