Add a not operator (~) for Sass booleans.

lib/sass/constant.rb

@@ -30,6 +30,7 @@ module Sass
       ?* => :times,
       ?/ => :div,
       ?% => :mod,
+      ?~ => :not,
       ?& => :single_and,
       ?| => :single_or,
       CONSTANT_CHAR => :const,

lib/sass/constant/literal.rb

@@ -53,6 +53,10 @@ class Sass::Constant::Literal # :nodoc:
     to_bool ? self : other
   end
 
+  def unary_not
+    Sass::Constant::Bool.from_value(!to_bool)
+  end
+
   def concat(other)
     Sass::Constant::String.from_value("#{self.to_s} #{other.to_s}")
   end

test/sass/engine_test.rb

@@ -396,6 +396,16 @@ a
   f2 = false && true
   f3 = true && false
   f4 = false && false
+SASS
+    assert_equal(<<CSS, render(<<SASS))
+a {
+  b: true;
+  c: false; }
+CSS
+!var = true
+a
+  b = ~~!var
+  c = ~!var
 SASS
   end