Description: Extend class notation to include unary minus. The symbol -u is not a
class by itself but part of a compound class definition. We do this
rather than making it a formal function since it is so commonly used.
Note: We use different symbols for unary minus ( -u ) and subtraction
cmin ( - ) to prevent syntax ambiguity. For example, looking at the
syntax definition co , if we used the same symbol
then " ( - A - B ) " could mean either " - A " minus " B ", or
it could represent the (meaningless) operation of
classes " - " and " - B " connected with "operation" " A ".
On the other hand, " ( -u A - B ) " is unambiguous.