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PublishedAPI for packages intended to be used by Plugin and Contrib authors, or 
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See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
 ObjectMethod ICON($params) → $html 
ICONURLPATH macro implementation
  
If the main parameter refers to a non-existent icon, and default is not
given, or also refers to a non-existent icon, then the else icon (else)
will be used. The HTML alt attribute for the image will be taken from
the alt parameter. If alt is not given, the main parameter will be used.