Recall from Chapter 6 that a referencing environment is a mapping that associates variable names (or symbols) with their current bindings at any point in a program in an implementation of a programming language (e.g., {(a, 4), (b, 2), (c, 3), (x, 5)}). Consider an interface specification of an environment, where formally an environment expressed in the mathematical form {(s1, v1), (s2, v2), . . . , (sn, vn)} is a mapping (or a set of pairs) from the domain— the finite set of Scheme symbols—to the range—the set of all Scheme values:
where {g(s′) = vi if s′ = si for some i, 1 ≤ i ≤ n, and f(s′) otherwise; ...
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