[Xy-pic] Change the orientation of Arrow of xtwocell

[Ross Moore]

On 25/08/2013, at 8:24 AM, Philip Hirschhorn wrote:

> If you don't mind ignoring the twocell method and experimenting a bit
> to aim the arrow correctly, you can just insert that double shafted
> arrow by having it point from the lower node to a spot displaced from
> the upper left hand node.  That is:
> 
>  \xymatrix{
>    {A} \ar[r]^{X} \ar[d]_{F}
>    & {C}\\
>    {B} \ar@{-->}[ur]_{R}
>    \ar@{=>} [u]+<1.7em,-0.5ex>
>  }
> 
> That arrow at the bottom goes from the B to a spot 1.7em to the right
> and 0.5ex below the node above.  It's crude (the 1.7em and -0.5ex were
> found by trial and error), but it might be what you want.

This quite a good approach, but it ties the displacement to
the size of the fonts, to get values for 'em' and 'ex'.
Also, the Arrow is implicitly attached to "B", whereas
it is logically a morphism from  R\circ F --> X .
There is no easy way to express this, except via \xtwocell
--- but it is hard to get this positioned correctly.

Attached is a summary of several solution attempts.
The 3rd shows how the positions of cells within an \xymatrix
can be accessed outside the matrix, to determine how the 
extra Arrow can be placed, using coordinates implicit 
within the diagram as a whole.
(Some trial and error is used to get good displacements.)


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> 
> Hope this helps,
> Phil Hirschhorn


Cheers

	Ross

------------------------------------------------------------------------
Ross Moore                                       ross.moore at mq.edu.au 
Mathematics Department                           office: E7A-206      
Macquarie University                             tel: +61 (0)2 9850 8955
Sydney, Australia  2109                          fax: +61 (0)2 9850 8114
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