Variables may be written as any sequence of alphanumeric
characters (including _) starting with either a capital letter or
_; e.g.:
X Value A A1 _3 _RESULT
If a variable is only referred to once in a clause, it does
not need to be named and may be written as an anonymous variable,
indicated by the underline character _. A clause may
contain several anonymous variables; they are all read and treated
as distinct variables.
A variable should be thought of as standing for some definite but unidentified object. This is analogous to the use of a pronoun in natural language. Note that a variable is not simply a writable storage location as in most programming languages; rather it is a local name for some data object, cf. the variable of pure LISP and identity declarations in Algol68.