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more complex strategies are possible.

There are basically two ways to access and manipulate the stack in a
C<PPCODE> block. First, by using the C<ST(i)> macro, to get, modify, or
replace the I<i>th item in the current stack frame, and secondly to push
(usually temporary) return values onto the stack. The first uses the
hidden C<ax> variable, which is set on entry to the XSUB, and is the index
of the base of the current stack frame. This remains unchanged throughout
execution of the XSUB. The second approach uses the local stack pointer,
C<SP> (more on that below), which on entry to the C<PPCODE> block points
to the base of the stack frame. Macros like C<mPUSHi()> store a temporary
SV at that location, then increment C<SP>. On return from a C<PPCODE>
XSUB, the current value of C<SP> is used to indicate to the caller how
many values are being returned.

In general these two ways of accessing the stack should not be mixed, or
confusion is likely to arise. The PUSH strategy is most useful when you
have no further use for the passed arguments, and just want to generate
and return a list of values, as in the C<one_to_n()> example above. The
C<ST(i)> strategy is better when you still need to access the passed
arguments. In the example below,