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.