[plt-scheme] union notation in helpdesk

Hi,

I've been looking at the documentation for the graph.ss library (in MrLib
7.5) and came across some notation that I am not familiar with;

 (add-links
 graph-snip<%><http://download.plt-scheme.org/doc/371/html/mrlib/mrlib-Z-H-47.html#node_tag_Temp_43>
 graph-snip<%><http://download.plt-scheme.org/doc/371/html/mrlib/mrlib-Z-H-47.html#node_tag_Temp_43>
  (union false? (is-a?/c
pen%<http://download.plt-scheme.org/doc/371/html/mred/mred-Z-H-546.html#node_tag_Temp_544>
))
  (union false? (is-a?/c
pen%<http://download.plt-scheme.org/doc/371/html/mred/mred-Z-H-546.html#node_tag_Temp_544>
))
  (union false? (is-a?/c
brush%<http://download.plt-scheme.org/doc/371/html/mred/mred-Z-H-420.html#node_tag_Temp_418>
))
  (union false? (is-a?/c
brush%<http://download.plt-scheme.org/doc/371/html/mred/mred-Z-H-420.html#node_tag_Temp_418>
))))      PROCEDURE

-- 
http://download.plt-scheme.org/doc/371/html/mrlib/mrlib-Z-H-55.html#node_sec_7.5

I can tell it's asking for things that implement either a class or matching
interface,

but the use of the 'union false?'
Is union the one from the parser tools collection?

I'm guessing that it returns #f if the interfce isn't implemented - or do I
have this all wrong.


Cheers,

Stephen
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