Help:Fancy diagram

$ \def\noalign#1{} \def\vskip#1pt{} \def\hidewidth{} $

The Diagram from [K] D. Knuth The $\rm \TeX{}$Book, p. 182:

$$ \def\normalbaselines{\baselineskip20pt\lineskip1pt\lineskiplimit0pt } \def\mapright#1{\smash{ \mathop{\longrightarrow}\limits^{#1}}} \def\mapdown#1{\Big\downarrow \rlap{\vcenter{\hbox{$\scriptstyle#1$}}}} %% line for MathJax as used above % \rlap{$\vcenter{\hbox{$\scriptstyle#1$}}$}} %% line for TeX; this is the line from [K] \matrix{\noalign{\vskip6pt}&&&&&&0\cr &&&&&&\mapdown{}\cr 0&\mapright{}&{\cal O}_C&\mapright\iota& \cal E&\mapright\rho&\cal L&\mapright{}&0\cr &&\Big\Vert&&\mapdown\phi&&\mapdown\psi\cr 0&\mapright{}&{\cal O}_C&\mapright{}& \pi_*{\cal O}_D&\mapright\delta& R^1f_*{\cal O}_V(-D)&\mapright{}&0\cr &&&&&&\mapdown{\theta_i\otimes\gamma^{-1}}\cr &&&&&&\hidewidth R^1f_*\bigl({\cal O} _V(-iM)\bigr)\otimes\gamma^{-1}\hidewidth\cr &&&&&&\mapdown{}\cr &&&&&&0\cr\noalign{\vskip6pt}} $$

Comment: The line in the code below starting with \rlap has to be different for MathJax and ${\rm \TeX}$.

$$
\def\normalbaselines{\baselineskip20pt\lineskip1pt\lineskiplimit0pt }
\def\mapright#1{\smash{
    \mathop{\longrightarrow}\limits^{#1}}}
\def\mapdown#1{\Big\downarrow
    \rlap{ \vcenter{\hbox{$\scriptstyle#1$}} }}      %% line for MathJax as used fore the display above
% \rlap{$\vcenter{\hbox{$\scriptstyle#1$}}$}}    %% line for TeX; this is the line from [K]
\matrix{\noalign{\vskip6pt}&&&&&&0\cr
  &&&&&&\mapdown{}\cr
  0&\mapright{}&{\cal O}_C&\mapright\iota&
    \cal E&\mapright\rho&\cal L&\mapright{}&0\cr
  &&\Big\Vert&&\mapdown\phi&&\mapdown\psi\cr
  0&\mapright{}&{\cal O}_C&\mapright{}&
    \pi_*{\cal O}_D&\mapright\delta&
    R^1f_*{\cal O}_V(-D)&\mapright{}&0\cr
  &&&&&&\mapdown{\theta_i\otimes\gamma^{-1}}\cr
  &&&&&&\hidewidth R^1f_*\bigl({\cal O}
    _V(-iM)\bigr)\otimes\gamma^{-1}\hidewidth\cr
  &&&&&&\mapdown{}\cr
  &&&&&&0\cr\noalign{\vskip6pt}}
$$