# xymatrix/diagram: changing arrow tips/tails with spacing

I want to typeset two different kinds of arrows, as in

$\xymatrix{ \cdot \ar@{{(}{-}{>}}[r]^a & \cdot \\ \cdot \ar@{{)}{-}{>}}[r]^a & \cdot }$


which produces

Object alignment is fine, of course, but I want the arrows to have the same length, too. That is, the left of ")" (red) should be vertically aligned to the left of "(" (green).

Does anybody know how to do it?

I know there are "similar", well-aligned tips and tails such as ^c and many others, and I also know, that TikZ is said to offer an easier way of redefining arrow tips. Sadly, it must be a xymatrix and it must be "(" and ")".

Any suggestion greatly appreciated!!!

Use a workaround similar to https://tex.stackexchange.com/a/177255/4427

\documentclass{article}
\usepackage[all,cmtip]{xy}

\newdir{ )}{{}*!/-0.6em/@{)}} % exercise 14

\begin{document}

$\xymatrix{ \cdot \ar@{(->}[r]^a & \cdot \\ \cdot \ar@{ )->}[r]^a & \cdot }$

\end{document}