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A variable is a string of upper-case letters, lower-case letters, digits and underscore characters that starts either with an upper-case letter or with underscore. For example, X, Y, Variable, _tag, X_526, and List, List24, _head, Tail, _input and Output are all Prolog variables.
The variable _ (that is, a single underscore character) is rather special. It's called the anonymous variable, and we discuss it in a later lecture.
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