I will provide a detailed solution of the last problem
for those who have perhaps missed the beginning.


Verify that S = B(BW)(BBC). That is verify that B(BW)(BBC)xyz = xz(yz).
This should be much easier.



One awful solution would be, reminding that B = S(KS)K, and that W = SS(KI)
and that C = S(BBS)(KK), that is S((S(KS)K)(S(KS)K)S)(KK), to substitute those
the occurence of B, W and C by they SK-programs, giving:

S = B(BW)(BBC) =
= S(KS)K((S(KS)K)SS(K(SKK)))((S(KS)K)(S(KS)K)(S((S(KS)K)(S(KS)K)S)(KK)))

... and then applying that _expression_ on xyz.

Obviously we have not programmed the elementary compositor B, the elementary
permutator C and the elementary duplicator W for not using them. Yet this gave an
interesting definition of S in term of itself and this rises the question if there is not
some general recursion phenomenon there and indeed there will be a very general
such phenomenon captured by a combinator which is neither an eliminator, nor a
permutator, nor a duplicator, nor even an identificator (like Ix = x) and which was
called the paradoxical combinators by Curry and Feys and is known today as the
fixed  point combinator but that's for later.

So I recall:

Bxyz = x(yz)
Wxy = xyy
Cxyz = xzy

And 'course:
Kxy = x
Sxyz = xz(yz)

And let us verify that S = B(BW)(BBC)
I recall "=" means "act similarly"; thus we must show that

B(BW)(BBC)xyz = Sxyz = xz(yz)

Well Bxyz = x(yz)  and please remember that
Bxyz abbreviates (((B x) y) z) and in particular

B(BW)(BBC)x abbreviates (((B (BW)) (BBC)) x) and that that matches!

Another practical way (more practical than by adding the left
parenthesis!) is to fully abbreviate the _expression_ (like I do usually) and
remember that B (here) is trigged by its dynamic when presented
with three arguments and that argument are *arbitrary* combinators:

so the _expression_ B(BW)(BBC)xyz is

B     (BW)      (BBC)         x       y       z
          1            2             3

and you can write the dynamic of B by  B123 = 1(23), meaning
that 1 denotes its first argument, 2 its second ..., so that

B(BW)(BBC)x gives (BW)((BBC)x) that is (fully abbreviated)

BW(BBCx).  We must yet apply it to y and then z:

BW(BBCx)y

Oh! here we have a choice!  Indeed the B-dynamic match the
first occurrence of B and the second one. A famous result,
known as Church Rosser Theorem tells us that as far as the
reduction will converge on some stable molecule, the path, and
thus those choice does not matter: we will get the same
stable molecule. Soon or later we will come back
on this, but let us just choose the leftmost reduction (another
theorem will make some advertising for that strategy, but
things will appear to be non trivial though ...). So we apply
the B-rule on leftmost B in:     B W (BBCx) y  giving
W((BBCx)y) that is (fully abbrev.) W(BBCxy), and now we have
also a choice: either we apply W(BBCxy) directly on z, or
we reduce it further. You could verify those alternate path as
exercise; let us apply W(BBCxy) directly on z. This gives
W(BBCxy)z (and Wab = abb) thus W(BBCxy)z gives
(BBCxy)zz = (fully abbrev.) BBCxyzz, and this gives
by the B-rule B123 = 1(23), (where 1, the first argument of B
is B itself, and 2 is C and 3 is x)  B(Cx)yzz which by the B
rule again (with 1 = (Cx), 2 = y, 3 = z) gives (Cx)(yz)z,
which by the C-rule C123 = 132 (with 1 = x, 2 = (yz)
3 = z gives xz(yz).   That is Sxyz and so we have shown that
B(BW)(BBC) behaves like S.

To sum up:   S = B(BW)(BBC) because
B(BW)(BBC)xyz
= BW(BBCx)yz
= W(BBCxy)z
= (BBCxy)zz
= B(Cx)yzz
= Cx(yz)z
= xz(yz)    which what Sxyz gives.



Of course the original exercise I gave was harder: program
S from B, W and C. It consisted in finding that B(BW)(BBC) or
something similar. But how could we have found such _expression_?
A nice thing is that the verification above, which just use the
dynamics of B, C and W gives us the answer: just copy the
execution above in the reverse order (cf programming here is
inverse execution). I do it and I comment:

?xyz    =  < B, C, ...>xyz  = xz(yz)

xz(yz)   so this is what we want as the
            result of B C W application
            on xyz. So we must transform
            xz(yz) as <B,C ..>xyz, that is
            get those final "x)y)z)", or
            xyz in fully abb. form.

Cx(yz)z  what a clever move! we are
            at once close to xyz, except
            that we have two parentheses
            too much, and one z to much.
            To suppress one z we will
            isolate it by moving the right
            parenthesis to the left. That's
            the inversion of the B-rule, so
            we arrive at:

B(Cx)yzz   and applying again the
                 B-rule, we get

(BBCxy)zz   I let some left parenthesis
                   so that the W-rule
                   applicability is highlightened

W(BBCxy)z   And now there just
                   remains a right parenthesis
                  we would like to push on the
                  left, which we can do by
                  two successive inverse B rule:
                  The first gives (with 1 = W, 2
                  = (BBCx) and 3 = y:

BW(BBCx)yz    the second gives
                     with 1 = (BW), 2 = (BBC)
                      and 3 = x:

B(BW)(BBC)xyz  we got what we wanted:

S = B(BW)(BBC). Both gives xz(yz) when
given x y and z (in that order!).


Question: is there a systematic method such
that giving any behavior like
 
  Xxyztuv = x(yx)(uvut)   <or what you want)

can generate systematically a corresponding
SK or BWCK -combinator?
The answer is yes. I let you meditate the question.
(This point will be made clear when we will meet
the terrible little cousins of the combinators
which are the *lambda _expression_*, (and which
in our context will just abbreviate complex
combinators), but I propose to progress
and make sure that the
SK combinator are Turing Universal.

I am not yet decided when I really should
introduce you to the paradoxical combinators,
which are rather crazy. Smullyan call
them wise birds, but I guess
it is an euphemism!

Mmhhh... Showing turing-universality
through the use of some paradoxical
combinator is easy (once we have defined
the numbers), but there is a risk you take
bad habits! Actually we don't need the
paradoxical combinators to prove the turing
universality of the SK forest, mmmmh...

Well actually I will be rather buzy so I give
you the definition of a paradoxical combinator
and I let you search one.

First show that for any combinator A there
is a combinator B such that AB = B.
B is called a fixed point of A. (like the center
of a wheel C is a fixed point of the rotations of
the wheel: RC = C). It is a bit amazing that
all combinators have a fixed point and that is
what I propose you try to show. Here are hints for
different arguments. 1) Show how to find a fixed
point of A (Arbitrary combinator) using just B, M
and A. (Mx = xx I recall). 2) The same using just the
Lark L (Lxy = x(yy) I recall).
Now, a paradoxical combinator Y is just a
combinator which applied on that A will give the
fixed point of A;  that is YA will give a B such that
AB = B, that is A(YA) gives YA, or more generally
Y is a combinator satisfying  Yx = x(Yx).



Bruno
============================
COMBINATORS I is
http://www.escribe.com/science/theory/m5913.html
COMBINATORS II is
http://www.escribe.com/science/theory/m5942.html
COMBINATORS III is
http://www.escribe.com/science/theory/m5946.html
COMBINATORS IV is
http://www.escribe.com/science/theory/m5947.html

COMBINATORS V is
http://www.escribe.com/science/theory/m5948.html

Resume:
Kxy = x
Sxyz = xz(yz)

http://iridia.ulb.ac.be/~marchal/

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