Given a pattern pat and an expression exp, (pat => exp) is a matching clause. The pattern pat and the expression exp are referred to as the guard and the body of the matching clause.
Given an expression exp0 and a sequence of matching clauses clseq, a case-expression can be formed as such: (case exp0 of clseq). To evaluate the case-expression under a given environment ENV0, we first evaluate exp0 under ENV0 to a value. If this value does not match the guard of any clause in clseq, then the evaluation of the case-expression aborts. Otherwise, we choose the first clause in clseq such that the value matches the guard of the clause. Let ENV1 be the environment that extends ENV0 with the bindings generated from this matching, and we evaluate the body of the chosen clause under ENV1. The value obtained from this evaluation is the value of the case-expression being evaluated.