\hrule width\hsize \vskip10pt
\begin{calstable}
\colwidths{{60pt}}
\brow
 \cell{a6} % a6
  \cell{b6} % b6
   \cell{c6} % c6
    \cell{d6} % d6
     \cell{e6} % e6
      \cell{f6} % f6
       \erow
\brow
 \cell{a5} % a5
  \nullcell{ltb} % b5
   \nullcell{tb} % c5
    \nullcell{rtb}\spancontent{b5, c5, d5} % d5
     \nullcell{ltr} % e5
      \cell{f5} % f5
       \erow
\brow
 \cell{a4} % a4
  \nullcell{ltr} % b4
   \nullcell{tl} % c4
    \nullcell{tr} % d4
     \nullcell{lr} % e4
      \cell{f4} % f4
       \erow
\brow
 \cell{a3} % a3
  \nullcell{lr} % b3
   \nullcell{lb} % c3
    \nullcell{rb}\spancontent{c3, c4, d3, d4} % d3
     \nullcell{lrb}\spancontent{e3, e4, e5} % e3
      \cell{f3} % f3
       \erow
\brow
 \cell{a2} % a2
  \nullcell{lrb}\spancontent{b2, b3, b4} % b2
   \nullcell{ltb} % c2
    \nullcell{tb} % d2
     \nullcell{tbr} % e2
      \spancontent{c2, d2, e2}\cell{f2} % f2
       \erow
\brow
 \cell{a1} % a1
  \cell{b1} % b1
   \cell{c1} % c1
    \cell{d1} % d1
     \cell{e1} % e1
      \cell{f1} % f1
       \erow
\end{calstable}