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Variables are available in three syntactic forms.
_
" and alphanumeric characters.
List_nums Head
_
", followed by an upper case letter, and then any sequence consisting of "_
" and alphanumeric characters.
_Dictionary _X_1
_
" alone denotes an anonymous variable. Repeated occurences of underscore in a query or rule denote different unnamed variables.
Variables beginning with an underscore should be used when there is just a single occurence of a variable in a rule. It suppresses the "single occurrence of variable" warning which is given otherwise, which is useful for picking up mis-typed variable names.