1. A computing system, comprising a processor and a memory in communication with the processor, the memory storing programming instructions executable by the processor to, from outside the body of a first closure having one or more entries that map one or more free variables to their respective bindings, access the content of the first closure and make a second closure, the making comprising: copying the body of the first closure, and modifying values to which free variables of the second closure are bound relative to the free variables of the first closure; wherein the making process preserves the identities of the free variables through the modifying step.