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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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