Correct handling of units.

width: 1cm - 5mm
=> width: 0.5cm (instead of -4cm)

@ratio: 100px/50px
=> @ratio == 2 (instead of 2px)

test/less/parens.less

@@ -3,7 +3,7 @@
   border: (@var * 2) solid black;
   margin: (@var * 1) (@var + 2) (4 * 4) 3;
   width: (6 * 6);
-  padding: 2px (6px * 6px);
+  padding: 2px (6 * 6px);
 }
 
 .more-parens {

test/less/variables.less

@@ -16,6 +16,7 @@
 
 @quotes: "~" "~";
 @q: @quotes;
+@onePixel: 1px;
 
 .variables {
   height: @b + @x + 0px; // 24px
@@ -49,12 +50,12 @@
     @name: 'var';
     name: @@name;
 }
+
 .alpha {
     @var: 42;
     filter: alpha(opacity=@var);
 }
 
-}
 // test current behaviour
 .polluteMixin() {
     @a: 'pollution';
@@ -62,4 +63,17 @@
 .testPollution {
     a: @a;
 	.polluteMixin();
-	a: @a;
+	a: @a;
+}
+
+.units {
+  width: @onePixel;
+  z-index: @onePixel / @onePixel;
+  square-pixel: @onePixel * @onePixel * @onePixel / @onePixel;
+  odd-unit: @onePixel * 4em / 2cm;
+  percentage: 10 * 50%;
+  pixels: 50px * 10;
+  conversion-metric-a: 20mm + 1cm;
+  conversion-metric-b: 1cm + 20mm;
+  conversion-imperial: 1in + 72pt + 6pc;
+}