The preceding example illustrates some rules for pattern matching:

1. Structure and constant parts of the pattern must match the input exactly.
   • (- ?x ?y) does not match (+ a b) .

   • The pattern (+ (- ?x) ?y) does not match the input (+ a b) .

2. Variables must match consistently. A variable in the pattern can match anything in the input the first time, but it must match the same thing thereafter.
   • The pattern (- ?x ?x)
     matches (- (sin theta) (sin theta))
     but not (- (sin theta) (cos theta)) .

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