\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}
\[
\renewcommand{\labelstyle}{\scriptstyle}
\renewcommand{\objectstyle}{\scriptstyle}
\knotholesize{3mm}
\renewcommand\Vcap[2]{\save 0;#2-#1:#1,
  \vcap~{#1+(0,1)}{#2+(0,1)}{#1}{#2}\restore}
\xygraph{!{0;/r5mm/:}
  !P3"t"{~>{.}~*{\xypolynode}}
  !P6"h"{~:{(4.5,0):}~>{.}~*{\xypolynode}}
  !{\xunderv~{"h2"}{"h1"}{"t1"}{"t3"}@(.62)}
  !{\xunderv~{"h4"}{"h3"}{"t2"}{"t1"}@(.62)}
  !{\xunderv~{"h6"}{"h5"}{"t3"}{"t2"}@(.62)}
  !{\Vcap{"h3"}{"h2"}\Vcap{"h5"}{"h4"}%
    \Vcap{"h1"}{"h6"}}
}\]
\end{document}