Better arg checking and error messages for the opacity functions.

context.cpp

@@ -124,11 +124,11 @@ namespace Sass {
     register_function(invert_sig, invert);
     // Opacity Functions
     register_function(alpha_sig, alpha);
-    register_function(opacity_sig, alpha);
+    register_function(opacity_sig, opacity);
     register_function(opacify_sig, opacify);
-    register_function(fade_in_sig, opacify);
+    register_function(fade_in_sig, fade_in);
     register_function(transparentize_sig, transparentize);
-    register_function(fade_out_sig, transparentize);
+    register_function(fade_out_sig, fade_out);
     // String Functions
     register_function(unquote_sig, unquote);
     register_function(quote_sig, quote);

functions.hpp

@@ -122,17 +122,23 @@ namespace Sass {
     // Opacity Functions ///////////////////////////////////////////////////
 
     extern Signature alpha_sig;
-    extern Signature opacity_sig;
     Node alpha(const Node, Environment&, Node_Factory&);
+
+    extern Signature opacity_sig;
+    Node opacity(const Node, Environment&, Node_Factory&);    
     
     extern Signature opacify_sig;
-    extern Signature fade_in_sig;
     Node opacify(const Node, Environment&, Node_Factory&);
+
+    extern Signature fade_in_sig;
+    Node fade_in(const Node, Environment&, Node_Factory&);
     
     extern Signature transparentize_sig;
-    extern Signature fade_out_sig;
     Node transparentize(const Node, Environment&, Node_Factory&);
 
+    extern Signature fade_out_sig;
+    Node fade_out(const Node, Environment&, Node_Factory&);
+
     // Other Color Functions ///////////////////////////////////////////////
     
     extern Signature adjust_color_sig;