# Arrow joining beginning and end of line with xy-pic

I'm trying to draw a sequence of arrows with xy-pic that extends over two lines, and so I need to draw an arrow joining the end of the first line with the beginning of the second line. Here's the code I've tried:

\documentclass{amsart}
\usepackage[all]{xy}

\begin{document}

$\xymatrix{ \cdots\ar[r] & \mathrm{K}_j(M^{p+1}) \ar[r]^{i^p_j} &\mathrm{K}_j(M^{p}) \ar[r]^{q^p_j} & \mathrm{K}_j(M^{p}/M^{p+1}) \arr[d] d[dl] l[ddlll]_{b^p_j} d[ddll] '[ddll]&\\ & & & &\\ & \mathrm{K}_{j-1}(M^{p+1}) \ar[r]^{i^p_{j-1}} &\mathrm{K}_{j-1}(M^{p}) \ar[r]^{q^p_{-1}} & \mathrm{K}_{j-1}(M^{p}/M^{p+1}) \ar[r] & \cdots }$

\end{document}


This produces the following output, which is almost what I want.

The problem is that the head of the arrow joining the first and the second line is not positioned correctly. I have a suspicion that this is due to me not understanding the syntax of xy-pic correctly, so I would be glad if someone could point out what I'm doing wrong.

Edit: I just realized that there is a typo in the above code: I typed an apostrophe instead of a backquote in front of the last target of the relevant arrow. The code should read

\documentclass{amsart}
\usepackage[all]{xy}

\begin{document}

$\xymatrix{ \cdots\ar[r] & \mathrm{K}_j(M^{p+1}) \ar[r]^{i^p_j} &\mathrm{K}_j(M^{p}) \ar[r]^{q^p_j} & \mathrm{K}_j(M^{p}/M^{p+1}) \arr[d] d[dl] l[ddlll]_{b^p_j} d[ddll] [ddll]&\\ & & & &\\ & \mathrm{K}_{j-1}(M^{p+1}) \ar[r]^{i^p_{j-1}} &\mathrm{K}_{j-1}(M^{p}) \ar[r]^{q^p_{-1}} & \mathrm{K}_{j-1}(M^{p}/M^{p+1}) \ar[r] & \cdots }$

\end{document}


which produces the following output.

This outcome is unfortunately even worse.

% arara: pdflatex
$\xymatrix{ \cdots\ar[r] & \mathrm{K}_j(M^{p+1}) \ar[r]^{i^p_j} &\mathrm{K}_j(M^{p}) \ar[r]^{q^p_j} & \mathrm{K}_j(M^{p}/M^{p+1}) \arr[d] d[dl] l[ddlll]_{b^p_j} d[ddll] [ddll] & \\ & & & &\\ & \mathrm{K}_{j-1}(M^{p+1}) \ar[r]^{i^p_{j-1}} &\mathrm{K}_{j-1}(M^{p}) \ar[r]^{q^p_{-1}} & \mathrm{K}_{j-1}(M^{p}/M^{p+1}) \ar[r] & \cdots }$
`