[Xy-pic] Question about building new arrow-heads

[Michael Abbott]

I fear the core xy language is incompatible with my brain.  So I'll just
ask for the answers, please, if anybody knows.

1.  I want an open headed arrow.  I'm currently using the definition in
the reference which is:

	\newdir{|>}{!/4.5pt/\dir{|}
	    *:(1,-.2)\dir^{>}
	    *:(1,+.2)\dir_{>}}

but of course this has the problem that the arrowhead is *centered* on the
target rather than ending on the target.  As a hack I use

	\newdir{|> }{{}*!/-10pt/@{|>}}

but this is stupid: I'd rather define \dir{|>} right.  I've tried hacks
along the lines of

	% This one is a total mess:
	\newdir{|>}{!/10pt/\dir{|}
	    *:(1,-.2)!/5pt/\dir^{>}
	    *:(1,+.2)!/5pt/\dir_{>}}

I'm clearly missing the point altogether here.


2.  I want an arrow that looks like this: --->|>, ie an open head arrow
with an arrow right behind it.  I can't figure out which variant of the
xy language is in play inside the \newdir{>|>}{...} environment, and
everything I try is one of: wrong, a syntax error, an infinite loop.
Sigh.

I can't see how to drop one component and then move on to the next without
disturbing the first drop.