# How to do this diagram using xymatrix?

How can I do a diagram like this

using xymatrix? I don't want the diagram to extend for three lines (otherwise I could do the abo above diagram easily).

\documentclass{report}
$\xymatrix@C=1cm{x\ar[r]^{a} & y} \qquad \xymatrix{\ar@{|->}[r]^{T} & } \qquad \begin{gathered} \xymatrix@C=1cm@R=1cm{F(x)\ar[r]^{F(a)}\ar[d] & F(y) \ar[d] \\ G(x)\ar[r]_{G(a)} & G(y) } \end{gathered}$