Closures are trivial: a structure with a function pointer as a first element (i.e., the structure alignment is function pointer-compatible) and the captured env

Closures are trivial: a structure with a function pointer as a first element (i.e., the structure alignment is function pointer-compatible) and the captured environment following it. Pointer to this structure is passed as the first argument. The only potentially funny bit with the closures is construction of a set of potentially mutually recursive closures - in such case you have to defer filling in the corresponding environment fields until all the closure structures are allocated.