\documentclass{ppex}
\nonstopmode
\usepackage [all,knot,poly,dvips]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all,knot,poly,dvips]{xy}
\renewcommand{\labelstyle}{\scriptstyle}
\renewcommand{\objectstyle}{\scriptstyle}
\[\xygraph{!{0;/r2mm/:}
  !P3"t"{~>{.}}
  !P6"h"{~:{(4.5,0):}~>{.}~*{\xypolynode}}
  !P12"d"{~:{(9,0):}~>{.}~*{\xypolynode}}
  !P12"D"{~:{(13.,0):}~>{.}~*{\xypolynode}}
  !{\xoverv~{"h2"}{"h1"}{"t1"}{"t3"}@(.62)}
  !{\xoverv~{"h4"}{"h3"}{"t2"}{"t1"}@(.62)}
  !{\xoverv~{"h6"}{"h5"}{"t3"}{"t2"}@(.62)}
  !{\vover~{"d4"}{"d3"}{"h3"}{"h2"}}
              ,"d4"-@`{"D5","D6"}"d7",
  !{\vover~{"d8"}{"d7"}{"h5"}{"h4"}}
              ,"d8"-@`{"D9","D10"}"d11",
  !{\vover~{"d12"}{"d11"}{"h1"}{"h6"}}
              ,"d12"-@`{"D1","D2"}"d3"
}\]
\end{document}