% Read in by FEYNMANDOC:  FEYNMANDOC4G.  Called by FD4G.COM
%\chapter{Advanced Features con't Links, Stems and Caps: STEMS}
%
\subsubsection{Stemmed Vertices}

The \bs{\it stemvertex} and \bs{\it stemvertices} qualifiers add a stem or stems
to the rear of vertex lines drawn with a \ddrawvertex\ command.
The syntax is:
\begin{verbatim}

\stemvertex<n>\drawvertex...
\stemvertices\drawvertex...

\end{verbatim}
where \verb@<n>@ is the number of the vertex line to be stemmed.  
\bs stemvertices stems all of the lines.  Theses commands remain in effect only
for the current \ddrawvertex.  For example
\begin{verbatim}

\stemvertex2\drawvertex\photon[\E 3](\Xone,\Yone)[6]
\global\stemlength=1415
\stemvertices\drawvertex\gluon[\SW 4](0,0)[4]

\end{verbatim}
will draw a three-photon vertex with a stem of standard length on the second
line (in the NE direction) and a four-gluon vertex with stems of length 1415 on
each of the four lines.  Note that the stemlength is not reset until the entire
vertex has been completed.  To stem just lines two and four one would have
written
\begin{verbatim}
\global\stemlength=1415
\stemvertex2\stemvertex4\drawvertex\gluon[\SW 4](0,0)[4]
\end{verbatim}
and so forth.  In this case \bs vertexonex,
\bs vertexoney \etc\ refer to the end of the stem and there is no easy
way of marking the end of a given line {\it sans stem}.
\bs stemvertex may be used in conjunction with \bs vertexlink and 
\bs vertexcap.  As an exercise try to produce:
\vskip 2.150in
\hskip 1.25in
\begin{picture}(8000,8000)
\stemvertex1\vertexlink3\drawvertex\gluon[\N 3](0,0)[4]
\drawline\fermion[\SW\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\SE\REG](\vertexonex,\vertexoney)[2000]
%\drawline\fermion[\SW\REG](\vertexthreex,\vertexthreey)[2000]
%\drawline\fermion[\NW\REG](\vertexthreex,\vertexthreey)[2000]
\drawline\fermion[\N\REG](\vertextwox,\vertextwoy)[2000]
\drawline\fermion[\W\REG](\vertextwox,\vertextwoy)[2000]
\global\stemlength=1500
\stemvertex3\vertexlink4\drawvertex\gluon[\NE 4](\vertexthreex,\vertexthreey)[3]
\drawline\fermion[\W\REG](\vertextwox,\vertextwoy)[2000]
\drawline\fermion[\N\REG](\vertextwox,\vertextwoy)[2000]
\drawline\fermion[\E\REG](\vertexthreex,\vertexthreey)[2000]
\drawline\fermion[\N\REG](\vertexthreex,\vertexthreey)[2000]
\stemlength=1500
\backstemmed\drawline\gluon[\SE\REG](\vertexfourx,\vertexfoury)[2]
\drawarrow[\SE\ATTIP](\pbackx,\pbacky)
\end{picture}
\vskip 0.45in
where the diagonal stems are of length 1500.  Use links and stems wherever
possible.