diff --git a/lib/coffee_script/grammar.js b/lib/coffee_script/grammar.js
index 30d17a61..b5b8ef90 100644
--- a/lib/coffee_script/grammar.js
+++ b/lib/coffee_script/grammar.js
@@ -381,9 +381,7 @@
     SimpleArgs: [o("Expression", function() {
         return $1;
       }), o("SimpleArgs , Expression", function() {
-        return ([$1].push($3)).reduce(function(a, b) {
-          return a.concat(b);
-        });
+        return $1 instanceof Array ? $1.concat([$3]) : [$1].concat([$3]);
       })
     ],
     // Try/catch/finally exception handling blocks.
diff --git a/lib/coffee_script/lexer.js b/lib/coffee_script/lexer.js
index 3070ad1d..f4413f75 100644
--- a/lib/coffee_script/lexer.js
+++ b/lib/coffee_script/lexer.js
@@ -1,5 +1,5 @@
 (function(){
-  var ASSIGNMENT, CALLABLE, CODE, COMMENT, COMMENT_CLEANER, HEREDOC, HEREDOC_INDENT, IDENTIFIER, JS, JS_CLEANER, KEYWORDS, LAST_DENT, LAST_DENTS, MULTILINER, MULTI_DENT, NOT_REGEX, NO_NEWLINE, NUMBER, OPERATOR, REGEX, Rewriter, STRING, STRING_NEWLINES, WHITESPACE, lex;
+  var ASSIGNMENT, BEFORE_WHEN, CALLABLE, CODE, COMMENT, COMMENT_CLEANER, HEREDOC, HEREDOC_INDENT, IDENTIFIER, JS, JS_CLEANER, KEYWORDS, LAST_DENT, LAST_DENTS, MULTILINER, MULTI_DENT, NOT_REGEX, NO_NEWLINE, NUMBER, OPERATOR, REGEX, Rewriter, STRING, STRING_NEWLINES, WHITESPACE, lex;
   Rewriter = require('./rewriter').Rewriter;
   // The lexer reads a stream of CoffeeScript and divvys it up into tagged
   // tokens. A minor bit of the ambiguity in the grammar has been avoided by
@@ -36,6 +36,8 @@
   NOT_REGEX = ['IDENTIFIER', 'NUMBER', 'REGEX', 'STRING', ')', '++', '--', ']', '}', 'FALSE', 'NULL', 'TRUE'];
   // Tokens which could legitimately be invoked or indexed.
   CALLABLE = ['IDENTIFIER', 'SUPER', ')', ']', '}', 'STRING'];
+  // Tokens that, when immediately preceding a 'WHEN', indicate that its leading.
+  BEFORE_WHEN = ['INDENT', 'OUTDENT', 'TERMINATOR'];
   // Scan by attempting to match tokens one character at a time. Slow and steady.
   lex.prototype.tokenize = function tokenize(code) {
     this.code = code;
@@ -101,7 +103,7 @@
     // Keywords are special identifiers tagged with their own name,
     // 'if' will result in an ['IF', "if"] token.
     tag = KEYWORDS.indexOf(id) >= 0 ? id.toUpperCase() : 'IDENTIFIER';
-    if (tag === 'WHEN' && (this.tag() === 'OUTDENT' || this.tag() === 'INDENT')) {
+    if (tag === 'WHEN' && BEFORE_WHEN.indexOf(this.tag()) >= 0) {
       tag = 'LEADING_WHEN';
     }
     if (tag === 'IDENTIFIER' && this.value() === '::') {
diff --git a/lib/coffee_script/parser.js b/lib/coffee_script/parser.js
index 54c2389e..f3881b88 100755
--- a/lib/coffee_script/parser.js
+++ b/lib/coffee_script/parser.js
@@ -307,9 +307,7 @@ case 148:this.$ = $$[$0-2+1-1];
 break;
 case 149:this.$ = $$[$0-1+1-1];
 break;
-case 150:this.$ = ([$$[$0-3+1-1]].push($$[$0-3+3-1])).reduce(function(a, b) {
-          return a.concat(b);
-        });
+case 150:this.$ = $$[$0-3+1-1] instanceof Array ? $$[$0-3+1-1].concat([$$[$0-3+3-1]]) : [$$[$0-3+1-1]].concat([$$[$0-3+3-1]]);
 break;
 case 151:this.$ = new TryNode($$[$0-3+2-1], $$[$0-3+3-1][0], $$[$0-3+3-1][1]);
 break;
@@ -525,4 +523,4 @@ exports.main = function commonjsMain(args) {
 if (require.main === module) {
 	exports.main(require("system").args);
 }
-}
+}
\ No newline at end of file
diff --git a/src/grammar.coffee b/src/grammar.coffee
index f0cceba8..671d91a7 100644
--- a/src/grammar.coffee
+++ b/src/grammar.coffee
@@ -335,7 +335,7 @@ grammar: {
   SimpleArgs: [
     o "Expression",                             -> $1
     o "SimpleArgs , Expression",                ->
-      ([$1].push($3)).reduce (a, b) -> a.concat(b)
+      if $1 instanceof Array then $1.concat([$3]) else [$1].concat([$3])
   ]
 
   # Try/catch/finally exception handling blocks.
@@ -384,8 +384,10 @@ grammar: {
   ForSource: [
     o "IN Expression",                          -> {source:   $2}
     o "OF Expression",                          -> {source:   $2, object: true}
-    o "ForSource WHEN Expression",              -> $1.filter: $3; $1
-    o "ForSource BY Expression",                -> $1.step:   $3; $1
+    o "ForSource WHEN Expression",              ->
+      $1.filter: $3; $1
+    o "ForSource BY Expression",                ->
+      $1.step:   $3; $1
   ]
 
   # Switch/When blocks.
diff --git a/src/lexer.coffee b/src/lexer.coffee
index 69ab3cfc..6d8a7b83 100644
--- a/src/lexer.coffee
+++ b/src/lexer.coffee
@@ -57,6 +57,9 @@ NOT_REGEX: [
 # Tokens which could legitimately be invoked or indexed.
 CALLABLE: ['IDENTIFIER', 'SUPER', ')', ']', '}', 'STRING']
 
+# Tokens that, when immediately preceding a 'WHEN', indicate that its leading.
+BEFORE_WHEN: ['INDENT', 'OUTDENT', 'TERMINATOR']
+
 # Scan by attempting to match tokens one character at a time. Slow and steady.
 lex::tokenize: (code) ->
   @code    : code       # Cleanup code by remove extra line breaks, TODO: chomp
@@ -94,7 +97,7 @@ lex::identifier_token: ->
   # Keywords are special identifiers tagged with their own name,
   # 'if' will result in an ['IF', "if"] token.
   tag: if KEYWORDS.indexOf(id) >= 0 then id.toUpperCase() else 'IDENTIFIER'
-  tag: 'LEADING_WHEN' if tag is 'WHEN' and (@tag() is 'OUTDENT' or @tag() is 'INDENT')
+  tag: 'LEADING_WHEN' if tag is 'WHEN' and BEFORE_WHEN.indexOf(@tag()) >= 0
   @tag(-1, 'PROTOTYPE_ACCESS') if tag is 'IDENTIFIER' and @value() is '::'
   if tag is 'IDENTIFIER' and @value() is '.' and !(@value(2) is '.')
     if @tag(2) is '?'
diff --git a/test/fixtures/execution/test_switch.coffee b/test/fixtures/execution/test_switch.coffee
index b97fa011..7c6dda51 100644
--- a/test/fixtures/execution/test_switch.coffee
+++ b/test/fixtures/execution/test_switch.coffee
@@ -16,6 +16,7 @@ result: switch num
 
 puts result
 
+
 func: (num) ->
   switch num
     when 2, 4, 6