Promises are a convenient way to defer a calculation until its result is actually needed, and to run such a calculation only once.
Return the value obtained from evaluating the expr in the given promise p. If p has previously been forced then its expr is not evaluated again, instead the value obtained at that time is simply returned.
force, an expr can call
forceagain on its own promise, resulting in a recursive evaluation of that expr. The first evaluation to return gives the value for the promise. Higher evaluations run to completion in the normal way, but their results are ignored,
forcealways returns the first value.