# xymatrix: Arrows badly placed when used with large nodes

I find that using xymatrix with large nodes results in awkwardly placed arrows:

MWE:

\documentclass{book}
\usepackage{xypic}
\begin{document}
\xymatrix{
\textbf{Natural Numbers}
\ar@/_/[r]  &\textbf{Integers} \ar@/_/[l]
}
\end{document}

Suggestions on placing these more elegantly would be gratefully received.

(If at all possible, I would prefer solutions that used xymatrix. I have a number of diagrams using xymatrix, most of which are unproblematic, and I would prefer not to have to convert them all to e.g. tikz.)

Note that the xymatrix manual has information on changing the target of an arrow, but not the source... and here it is the source ends that are problematic.

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