Variables may be written as any sequence of alphanumeric characters
(including _) beginning with either a capital letter or _.
For example:
X Value A A1 _3 _RESULT
If a variable is referred
to only once in a clause, it does not need to be named and may be
written as an anonymous variable, represented by the underline
character _ by itself. Any number of anonymous variables may
appear in a clause; they are read as distinct variables. Anonymous
variables are not special at runtime.