Updated stack (markdown)

stack.md

@@ -30,5 +30,7 @@ Past Lisps have implemented stack as lists and as vectors. Both work. My own gue
 
 Note that every argument to a Lisp function must be a cons space object passed by reference (i.e., a cons pointer). If the actual argument is actually a [[vector space]] object, then what we pass is a reference to the VECP object which references that vector.
 
-I'm not certain we need a prior frame pointer; if we don't, we may not need a VECP pointing to a stack frame, since nothing can point to a stack frame other than the next stack frame(s) up the stack (if we have a *fork* function, or if we parallelise *map*) which to implement a multi-thread system we essentially must have, there may  be two or more successor frames to any frame.
+I'm not certain we need a prior frame pointer; if we don't, we may not need a VECP pointing to a stack frame, since nothing can point to a stack frame other than the next stack frame(s) up the stack (if we parallelise *map*, *and* and so on) which to implement a multi-thread system we essentially must have, there may  be two or more successor frames to any frame. In fact to use a massively multiprocessor machine efficiently we must normally evaluate each parameter in a separate thread, with only special forms such as *cond* which impose explicit control flow evaluating their clauses serially in a single thread.
+
+However, modern systems with small numbers of processors and expensive thread construction and tear-down would perform **terribly** if all parameter evaluation was parallelised, so for now we can't do that, even though the semantics must be such that later we can.