Make an uncomposed icon rule use all the icons from previous composed icon rules. Lets you express things like "compose icon a if XXX, compose icon b if YYY, icon c if ZZZ, icon d if WWW". This will result in categories icons a and b if their conditions match, then appended with one of: c if its condition matches, d if its condition matches, or nothing.

src/calibre/gui2/library/models.py

@@ -94,11 +94,9 @@ class ColumnIcon(object):  # {{{
                 if not rule_icons:
                     continue
                 icon_list = [ic.strip() for ic in rule_icons.split(':')]
-                if icon_list and not kind.endswith('_composed'):
-                    icons = icon_list
-                    break
-                else:
                 icons.extend(icon_list)
+                if icon_list and not kind.endswith('_composed'):
+                    break
 
             if icons:
                 icon_string = ':'.join(icons)