Using Tikz-cd "to path=" to Illustrate Composition of Natural Transformations

Question

I'm having two issues in Tikz-cd in trying to make the following picture:

which appears in Categories for the Working Mathematician by Saunders Mac Lane. I've looked at the documentation for Tikz-cd, and their example isn't very helpful.

My code so far:

    \begin{tikzcd}[row sep = 1.4cm, column sep = 1.4cm]
        F(c)
        \arrow[lddr, to path= { --
        ([xshift=-1ex]\tikztostart.west)
        -| ([xshift=-2ex]\tikztotarget.west)
        |- (\tikztotarget)}]
        \arrow[d, swap, "\sigma(c)"]
        \arrow[r, "F(f)"] 
        & F(c')
        \arrow[d, "\sigma(c')"]
        \arrow[rddl, to path= { --
        ([xshift=1ex]\tikztostart.east) 
        -| ([xshift=2ex]\tikztotarget.east)
        -- (\tikztotarget)}, "\sigma \circ \tau(c')"]
        \\
        G(c)
        \arrow[d, swap, "\tau(c)"] 
        \arrow[r, "G(f)"] & G(c')
        \arrow[d, "\tau(c')"] \\
        H(c) 
        \arrow[r, "H(f)"] & H(c')
    \end{tikzcd}    

which generates:

My problem: In my diagram, the outside arrows (i.e., the one starting at F(c) and ending at H(c) and the other at F(c') and ending at H(c')) are too close to the diagram. Is it possible to horizontally shift them?

It's okay if it's not, but I'd at least like to add some nodes to the outside arrows. I can't seem to do that. I can do this the hard way by putting the diagram in a Tikzpicture node and positioning the desired labels around the diagram, but I'd like to learn the Tikz-cd way. I know it's possible, since the documentation says so, it just barely says how.

Documentation I'm referring to (page 12): http://ctan.math.washington.edu/tex-archive/graphics/pgf/contrib/tikz-cd/tikz-cd-doc.pdf

Answer 1

Basically, I added node[near end, left]{} and node[near end, right]{} to insert the labels and fixed the values of xshift to be symetric, eg, +/- 3ex.

\documentclass[border=1mm]{standalone}
\usepackage{amsthm,amsmath,amssymb,amsfonts}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[row sep = 1.4cm, column sep = 1.4cm]
    F(c)
    \arrow[lddr, to path= {%
     -|  ([xshift=-3ex]\tikztotarget.west)node[near end,left]{$\tau\cdot\sigma(c)$}
      |- (\tikztotarget)}]
    \arrow[d, swap, "\sigma(c)"]
    \arrow[r, "F(f)"] 
  & F(c')
    \arrow[d, "\sigma(c')"]
    \arrow[rddl, to path= {%
     -| ([xshift=3ex]\tikztotarget.east)node[near end,right]{$\tau\cdot\sigma(c')$}
     -- (\tikztotarget)}]
  \\
    G(c)
    \arrow[d, swap, "\tau(c)"] 
    \arrow[r, "G(f)"] 
  & G(c')
    \arrow[d, "\tau(c')"]
  \\
    H(c) 
    \arrow[r, "H(f)"]
  & H(c')
\end{tikzcd} 
\end{document}