diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index d4a0d3f..4384168 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -76,33 +76,7 @@ module Rscons
     end
 
     def execute(short_desc, command, extra_vars)
-      merged_variables = @variables.merge(extra_vars)
-      expand_varref = proc do |varref|
-        if varref.is_a?(Array)
-          varref.map do |ent|
-            expand_varref.call(ent)
-          end
-        else
-          if varref =~ /^(.*)\$\[(\w+)\](.*)$/
-            # expand array with given prefix, suffix
-            prefix, varname, suffix = $1, $2, $3
-            varval = merged_variables[varname]
-            unless varval.is_a?(Array)
-              raise "Array expected for $#{varname}"
-            end
-            varval.map {|e| "#{prefix}#{e}#{suffix}"}
-          elsif varref =~ /^\$(.*)$/
-            # expand a single variable reference
-            varname = $1
-            varval = merged_variables[varname]
-            varval or raise "Could not find variable #{varname.inspect}"
-            expand_varref.call(varval)
-          else
-            varref
-          end
-        end
-      end
-      command = expand_varref.call(command.flatten).flatten
+      command = @variables.merge(extra_vars).expand_varref(command)
       if @variables[:echo] == :command
         puts command.map { |c| c =~ /\s/ ?  "'#{c}'" : c }.join(' ')
       elsif @variables[:echo] == :short
diff --git a/lib/rscons/varset.rb b/lib/rscons/varset.rb
index 8ce60a1..74fdbce 100644
--- a/lib/rscons/varset.rb
+++ b/lib/rscons/varset.rb
@@ -21,9 +21,41 @@ module Rscons
       @vars[key] = val
     end
 
-    def merge(other)
-      other = other.vars if other.is_a?(VarSet)
-      VarSet.new(@vars.merge(other))
+    def append(values)
+      values = values.vars if values.is_a?(VarSet)
+      @vars.merge!(values)
+      self
+    end
+
+    def merge(other = {})
+      VarSet.new(Marshal.load(Marshal.dump(@vars))).append(other)
+    end
+    alias_method :clone, :merge
+
+    def expand_varref(varref)
+      if varref.is_a?(Array)
+        varref.map do |ent|
+          expand_varref(ent)
+        end.flatten
+      else
+        if varref =~ /^(.*)\$\[(\w+)\](.*)$/
+          # expand array with given prefix, suffix
+          prefix, varname, suffix = $1, $2, $3
+          varval = @vars[varname]
+          unless varval.is_a?(Array)
+            raise "Array expected for $#{varname}"
+          end
+          varval.map {|e| "#{prefix}#{e}#{suffix}"}
+        elsif varref =~ /^\$(.*)$/
+          # expand a single variable reference
+          varname = $1
+          varval = @vars[varname]
+          varval or raise "Could not find variable #{varname.inspect}"
+          expand_varref(varval)
+        else
+          varref
+        end
+      end
     end
   end
 end