In logic programming, a substitution is a mapping from logic variables to values (including other logic variables). Logic programs are composed of goals, with the type signature Subst => Subst* — i.e. it might fail (thus generating 0 possible substitution), succeed once, or succeed multiple times, or even an infinite number of times.
It is necessary for substitutions to behave as values — if you take a substitution, and add a binding, you better get a new substitution, rather than do the modification in place, because that substitution might be used by other goals. In Lisp, the traditional data structure for this is the association list: a list of lists, where the cars (first elements) of each inner list are the keys. Clojure takes Phil Bagwell’s Hash Array Mapped Trie and adds persistency on top of it, to the effect that you have a copy-on-write trie map, with logarithmic rather than linear lookup times.
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Intuitionistically Uncertain 