--- a/DeviceGraphicsContext.st	Wed Mar 07 17:55:09 2018 +0100
+++ b/DeviceGraphicsContext.st	Wed Mar 07 18:06:42 2018 +0100
@@ -487,14 +487,19 @@
     "Modified: / 22-10-2006 / 14:11:37 / cg"
 !
 
-function:aSymbol
-    "set the drawing function if it has changed"
-
-    (aSymbol ~~ function) ifTrue:[
-	function := aSymbol.
-	gcId notNil ifTrue:[
-	    device setFunction:aSymbol in:gcId
-	]
+function:functionSymbol
+    "set the drawing function if it has changed.
+     functionSymbol is one of:
+            #copy,#copyInverted,#xor,#and,#andReverse
+            #andInverted,#or,#orReverse,#orInverted
+            #invert,#clear,#set,#noop,#equiv,#nand.
+     Notice: not all graphicMedia support all functions"
+
+    (functionSymbol ~~ function) ifTrue:[
+        function := functionSymbol.
+        gcId notNil ifTrue:[
+            device setFunction:functionSymbol in:gcId
+        ]
     ]
 !