+ The calling convention is that a caller prepares a stack frame
+ consisting of the saved FP and the return address, followed by the
+ procedure and then the arguments to the call, in order. Thus in the
+ beginning of a call, the procedure being called is in slot 0, the
+ first argument is in slot 1, and the SP points to the last argument.
+ The number of arguments, including the procedure, is thus SP - FP +
+ 1.
+
+ After ensuring that the correct number of arguments have been passed,
+ a function will set the stack pointer to point to the last local
+ slot. This lets a function allocate the temporary space that it
+ needs once in the beginning of the call, instead of pushing and
+ popping the stack pointer during the call's extent.
+
+ When a program returns, it returns its values in the slots starting
+ from local 1, as if the values were arguments to a tail call. We
+ start from 1 instead of 0 for the convenience of the "values" builtin
+ function, which can just leave its arguments in place.
+
+ The callee resets the stack pointer to point to the last value. In
+ this way the caller knows how many values there are: it's the number
+ of words between the stack pointer and the slot at which the caller
+ placed the procedure.
+
+ After checking that the number of values returned is appropriate, the
+ caller shuffles the values around (if needed), and resets the stack
+ pointer back to its original value from before the call. */