[Xy-pic] Delicate alignment problem.

[Michael Abbott]

>  What is wrong with using   *+!L{{}=BBBB}="BB"   to get the margin ?
>
> (which is different to  *!L+{{}=BBBB}="BB"  by the way)
> Does the extra slight shift to the right upset the diagram ?

Alas, yes, that's the problem.  I've tried that one, and it creates an
unwanted extra gap between AAAA and the =.

Obviously I'm trying to solve a generic problem here.  In this particular
case the problem is one of placing two boxes side by side while still
retaining full independent command over the resulting boxes.

Even if we fixed this with an extra sideways shift, eg
	-\whatever *+!L{{}=BBBB}="BB"
because the centre point is still in the wrong place the generic problem
remains open.  For example, I might like to draw the following:

	        |
	        v
	AAAA = BBBB
         ^
	 |

If I can't give BBBB back its proper centre then I can't do the right hand
arrow.

(What should \whatever be in the above, by the way?  Obviously it should
be an offset equal to the value added on each side by +.)