R.P.I. Lewis xypic at rtf.org.uk
Thu May 8 14:12:48 CEST 2003

```Michael Abbott <michael at araneidae.co.uk> writes:

> On Wed, 7 May 2003, Michael Barr wrote:
>> Here is a copy of the note I sent to Michael Abbott in response to his
>> recent request.
>
> Ah.  I guess I'd better post my reply to Michael Barr together with some
>
> Although Michael's answer is very interesting, I don't think it solves the
> problem I'm trying to tackle.  In particular, I'm really more interested
> in getting to grips with the underlying xy language.  It seems to be a
> slippery thing: each time I think I understand it, the next thing I try
> breaks :(
>

This sint as good as Michael's version, because the g arrow out of
AAAA isnt quite right, and nor is the spacing of the CCC\simeqDDD
stuff

\xymatrix at C+2cm{ **[r]{AAAA=BBBB}
\ar[r]^<>(.5)f
\ar[d]^<>(.5)g
& *+\txt<1cm>{\$CCC\$\\\$\simeq\$\\\$DDD\$}
\ar[d]^<>(.5)h\\
FFFF
& EEEE
\ar[l]^<>(.5)k}

Here the **[r] pushes the AAAA=BBBB node to the right (see exercise 31
of the xy reference manual)

The <>(.5) just make the label be halfway along the arrow, ratehr than
halfway between the centers of the objects.

In fact we can borrow the \phantom idea:

\xymatrix at C+2cm{ *+{\phantom{=BBBB}AAAA=BBBB}
\ar[r]^<>(.5)f
\ar[d]^<>(.5)g
& *+\txt<1cm>{\$CCC\$\\\$\simeq\$\\\$DDD\$}
\ar[d]^<>(.5)h\\
FFFF
& EEEE
\ar[l]^<>(.5)k}

I dont know how to adjust the vertical alignment of the
\$CCC\$\\\$\simeq\$\\\$DDD\$ node tho

```