The KnowledgeWorks Backward Chainer indexes clauses for a backward rule based on the first argument. If the first arguments to backward rule clauses are distinct non-variables, the backward chainer can pre-select possible matching clauses for a call.
For example, in the following rule:
(defrule age-of :backward
((age-of charlie 30) <--)
((age-of william 25) <--)
((age-of james 28) <--))
The call: (age-of james ?x) would jump directly to the third clause and bind ?x to 28 without trying the other two.
The call: (age-of tom ?x) would fail immediately without doing any pattern matching.
Clauses are distinguished first by the types and then the values of their first arguments.