[Xy-pic] equal arrow length

[Adriana Balan]

Hello,
I would appreciate some help and advice with the following problem: how to draw 
arrows of equal length, independently of their source and target? As a reference 
example, consider the following:
\documentclass{article}

\usepackage{amssymb}
\usepackage{amsmath}
\usepackage[all]{xy}
\begin{document}
\begin{equation*} 
\xymatrix at R=0pt at C=0.5in{(1+X\times \Bbbk)^{\infty} \ar[r]^{\lambda_X} & 
1+X^{\infty}\times \Bbbk \\
[-] \ar[r] & ([-],e) \\ 
\sigma=(\sigma_0,\sigma_1,\ldots) \ar[r] & ((x_0,x_1,\ldots),a_0a_1\ldots) 
\textrm{ where } \sigma_n=(x_n,a_n) 

}
\end{equation*}
\end{document}I would like to have the above looking better than now, with 
vertically aligned arrows of equal length.  

Thank you very much,
Adriana Balan


      
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