The -O option

Recently I've been trying to improve the performance
of some Prolog code.  One part of it manipulated
subsets of a small known universe as lists.  I decided
to replace this by bitwise operations.  For example,
	Set /\ (Set - 1) =\= 0
lets you test whether a set has 2 or more elements.
To my astonishment, this made the code take *more*
time and *more* memory.

Then the light-bulb went on.  "I need the -O option!"

That made all the difference: the code now ran 30%
faster, allocating 30% less memory.

As long as version 7 is making major changes to the
core language, this might be a good time to make
the -O option the default.

I do appreciate that SWI wants to recover the original
term representing a clause, but it could be done like
this:

	begin_fast_arithmetic L1
	<optimised code>
	end_fast_arithmetic L2
   L1:  <unoptimised code>
   L2:

The execution engine can treat begin_fast_arithmetic
as a no-nop and end_fast_arithmetic as an unconditional
branch, while the decompiler can do it the other way
around.