[plt-scheme] union notation in helpdesk

Stephen De Gabrielle wrote:
> 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;

> I can tell it's asking for things that implement either a class or 
> matching interface,
> 
> but the use of the 'union false?'

  (union false? (is-a?/c brush%))

Think of types as sets of values (belonging to the type).
The type (union false? (is-a?/c brush%)) consists
of the union of the values specified by false? and
(is-a?/c brush%). That is, it just means the false value
or a brush% (as you guessed).

-- 
Jens Axel Søgaard