[Xy-pic] A possible bug in labels for the arrow extension? (fwd)

Ross Moore ross at ics.mq.edu.au
Sat Aug 30 10:06:37 CEST 2003

Hello Ben

On Fri, 29 Aug 2003, Ben Whale wrote:

> Hi everyone,
>   So here is the troublesome xy-pic code.
> \ar[ldd]|!{[lld];[rd]}{\quad} doesn't insert \quad wher I want it to,

Sure it does.
What are you wanting to see?

> however, \ar[ldd]|!{[lld];[rd]}{d \quad d} will put the \quad in the right
> place (along with the unwanted characters).  What's going on here?

An object, such as  \drop{\quad}  would have a bounding rectangle
that has width but zero height.
Any line/arrow approaching it would come infinitely close from
above and from below. Any test of whether a point lies inside
the rectangle would always fail. Hence this kind of object cannot
cause a visible break in an arrow --- but it *is* there, you just
cannot see it.

Xypic defines a macro  \hole  (and \khole for knots)
for creating a small blank rectangle of size suitable
to break arrows.
Its size was chosen to be particularly good for creating a gap
to allow one arrow to cross another.

Hope this helps,

	Ross Moore

> Ben Whale
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