
In some cases, one is not interested in all constituents of a term. For example, if we wish to process the head of a list but without consisdering the tail, we may write:
'rule' ProcessHead(list(Head, Tail)):
ProcessColor(Head)
In this rule, two variables are defined in a pattern
( Head and Tail),
but only one of them is applied in an expression ( Head),
In such cases, it is not necessary to invent a name for a variable
that is not used (here Tail);
instead, we can use a joker, which is written as an underscore
(``_'') and matches any value. Hence, the above rule can be rewritten:
'rule' ProcessHead(list(Head, _)):
ProcessColor(Head)
