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.

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