diff --git a/lib/grammar.js b/lib/grammar.js
index 8cebd996..57a11693 100644
--- a/lib/grammar.js
+++ b/lib/grammar.js
@@ -130,7 +130,7 @@
       }), o("Param", function() {
         return [$1];
       }), o("ParamList , Param", function() {
-        return $1.concat([$3]);
+        return $1.concat($3);
       })
     ],
     Param: [
@@ -209,9 +209,9 @@
       }), o("AssignObj", function() {
         return [$1];
       }), o("AssignList , AssignObj", function() {
-        return $1.concat([$3]);
+        return $1.concat($3);
       }), o("AssignList OptComma TERMINATOR AssignObj", function() {
-        return $1.concat([$4]);
+        return $1.concat($4);
       }), o("AssignList OptComma INDENT AssignList OptComma OUTDENT", function() {
         return $1.concat($4);
       })
@@ -329,9 +329,9 @@
       o("Arg", function() {
         return [$1];
       }), o("ArgList , Arg", function() {
-        return $1.concat([$3]);
+        return $1.concat($3);
       }), o("ArgList OptComma TERMINATOR Arg", function() {
-        return $1.concat([$4]);
+        return $1.concat($4);
       }), o("INDENT ArgList OptComma OUTDENT", function() {
         return $2;
       }), o("ArgList OptComma INDENT ArgList OptComma OUTDENT", function() {
@@ -341,7 +341,7 @@
     Arg: [o("Expression"), o("Splat")],
     SimpleArgs: [
       o("Expression"), o("SimpleArgs , Expression", function() {
-        return $1 instanceof Array ? $1.concat([$3]) : [$1].concat([$3]);
+        return [].concat($1, $3);
       })
     ],
     Try: [
diff --git a/lib/parser.js b/lib/parser.js
index 61d420a6..4f6c106c 100755
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -133,7 +133,7 @@ case 61:this.$ = [];
 break;
 case 62:this.$ = [$$[$0-1+1-1]];
 break;
-case 63:this.$ = $$[$0-3+1-1].concat([$$[$0-3+3-1]]);
+case 63:this.$ = $$[$0-3+1-1].concat($$[$0-3+3-1]);
 break;
 case 64:this.$ = new yy.Literal($$[$0-1+1-1]);
 break;
@@ -199,9 +199,9 @@ case 91:this.$ = [];
 break;
 case 92:this.$ = [$$[$0-1+1-1]];
 break;
-case 93:this.$ = $$[$0-3+1-1].concat([$$[$0-3+3-1]]);
+case 93:this.$ = $$[$0-3+1-1].concat($$[$0-3+3-1]);
 break;
-case 94:this.$ = $$[$0-4+1-1].concat([$$[$0-4+4-1]]);
+case 94:this.$ = $$[$0-4+1-1].concat($$[$0-4+4-1]);
 break;
 case 95:this.$ = $$[$0-6+1-1].concat($$[$0-6+4-1]);
 break;
@@ -277,9 +277,9 @@ case 130:this.$ = new yy.ArrayLiteral($$[$0-4+2-1]);
 break;
 case 131:this.$ = [$$[$0-1+1-1]];
 break;
-case 132:this.$ = $$[$0-3+1-1].concat([$$[$0-3+3-1]]);
+case 132:this.$ = $$[$0-3+1-1].concat($$[$0-3+3-1]);
 break;
-case 133:this.$ = $$[$0-4+1-1].concat([$$[$0-4+4-1]]);
+case 133:this.$ = $$[$0-4+1-1].concat($$[$0-4+4-1]);
 break;
 case 134:this.$ = $$[$0-4+2-1];
 break;
@@ -291,7 +291,7 @@ case 137:this.$ = $$[$0-1+1-1];
 break;
 case 138:this.$ = $$[$0-1+1-1];
 break;
-case 139:this.$ = $$[$0-3+1-1] instanceof Array ? $$[$0-3+1-1].concat([$$[$0-3+3-1]]) : [$$[$0-3+1-1]].concat([$$[$0-3+3-1]]);
+case 139:this.$ = [].concat($$[$0-3+1-1], $$[$0-3+3-1]);
 break;
 case 140:this.$ = new yy.Try($$[$0-2+2-1]);
 break;
diff --git a/src/grammar.coffee b/src/grammar.coffee
index bf805547..c38603db 100644
--- a/src/grammar.coffee
+++ b/src/grammar.coffee
@@ -195,7 +195,7 @@ grammar =
   ParamList: [
     o "",                                       -> []
     o "Param",                                  -> [$1]
-    o "ParamList , Param",                      -> $1.concat [$3]
+    o "ParamList , Param",                      -> $1.concat $3
   ]
 
   # A single parameter in a function definition can be ordinary, or a splat
@@ -265,8 +265,8 @@ grammar =
   AssignList: [
     o "",                                       -> []
     o "AssignObj",                              -> [$1]
-    o "AssignList , AssignObj",                 -> $1.concat [$3]
-    o "AssignList OptComma TERMINATOR AssignObj", -> $1.concat [$4]
+    o "AssignList , AssignObj",                 -> $1.concat $3
+    o "AssignList OptComma TERMINATOR AssignObj", -> $1.concat $4
     o "AssignList OptComma INDENT AssignList OptComma OUTDENT", -> $1.concat $4
   ]
 
@@ -363,8 +363,8 @@ grammar =
   # (i.e. comma-separated expressions). Newlines work as well.
   ArgList: [
     o "Arg",                                    -> [$1]
-    o "ArgList , Arg",                          -> $1.concat [$3]
-    o "ArgList OptComma TERMINATOR Arg",        -> $1.concat [$4]
+    o "ArgList , Arg",                          -> $1.concat $3
+    o "ArgList OptComma TERMINATOR Arg",        -> $1.concat $4
     o "INDENT ArgList OptComma OUTDENT",        -> $2
     o "ArgList OptComma INDENT ArgList OptComma OUTDENT", -> $1.concat $4
   ]
@@ -380,8 +380,7 @@ grammar =
   # having the newlines wouldn't make sense.
   SimpleArgs: [
     o "Expression"
-    o "SimpleArgs , Expression",                ->
-      if $1 instanceof Array then $1.concat([$3]) else [$1].concat([$3])
+    o "SimpleArgs , Expression",                -> [].concat $1, $3
   ]
 
   # The variants of *try/catch/finally* exception handling blocks.
@@ -527,8 +526,8 @@ grammar =
   # rules are necessary.
   Operation: [
     o "UNARY Expression",                       -> new Op $1, $2
-    o("- Expression",                           (-> new Op('-', $2)), {prec: 'UNARY'})
-    o("+ Expression",                           (-> new Op('+', $2)), {prec: 'UNARY'})
+    o "- Expression",                          (-> new Op '-', $2), prec: 'UNARY'
+    o "+ Expression",                          (-> new Op '+', $2), prec: 'UNARY'
 
     o "-- Expression",                          -> new Op '--', $2
     o "++ Expression",                          -> new Op '++', $2