Custom commands for `xymatrix` paths

Question

I would like to define a custom command to replace \ar@{->}`r[d]`[l]`[llld]_-{\delta}`[d][lld] in this bit of code:

\[\xymatrix{
\cdots\ar@{->}[r] & H_n(X)\ar@{->}[r] & H_n(Y)\ar@{->}[r]
    & H_n(Z)\ar@{->}`r[d]`[l]`[llld]_-{\delta}`[d][lld] & \\
& H_{n-1}(X)\ar@{->}[r] & H_{n-1}(Y)\ar@{->}[r] & H_{n-1}(Z)\ar@{->}[r] & \cdots
}\]

I've tried \newcommand{\snake}[1]{\ar@{->}`r[d]`[l]`[llld]_-{#1}`[d][lld]}, but I keep getting errors with that. Does anyone know how to make custom commands to define xymatrix paths?

Answer 1

Your example works for me. Please try the following code:

% arara: pdflatex

\documentclass{article}
\usepackage[all, pdf, cmtip]{xy}
\newcommand{\snake}[1]{\ar@{->}`r[d]`[l]`[llld]_-{#1}`[d][lld]}

\begin{document}
\[\xymatrix{
\cdots\ar@{->}[r] & H_n(X)\ar@{->}[r] & H_n(Y)\ar@{->}[r]
    & H_n(Z)\snake{\delta} & \\
& H_{n-1}(X)\ar@{->}[r] & H_{n-1}(Y)\ar@{->}[r] & H_{n-1}(Z)\ar@{->}[r] & \cdots
}\]
\end{document}