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, then 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.