$$
\joined \let\labelstyle\textstyle
\Graph{3cm,2}{3cm,4}
\Line (0,0) (0,4) <{f(z,s)}
\Line (0,4) (2,4) ^{H(z,t)}
\Line (0,0) (2,0) _{*}
\Line (2,0) (2,4)
\Line (0,4) (2,2) _{\;\;H(z,0) = f(z,1)} \ro
\endGraph
\qquad\qquad
H'(z,t,s) =
\cases{f(z,2s/(2-t))& if $0\le 2s\le 2 - t$\cr
       \cr
       H(z,2s - 2 + t)& if $2 - t\le 2s\le 2$\cr}
$$