operator	usemap is

Return type: Boolean
Returns:       

True if the test returns #none or #empty (whichever was specified) or false otherwise.

  usemap (is | isnt)  (#none | #empty)

A test that identifies whether an element has had a short reference map attached to it by usemap declarations.

Specifying usemap isnt will reverse the results, and thus return false if the test returns #none or #empty (whichever was specified) or true otherwise.