% Read in by FEYNMANDOC:  FEYNMANDOC4B.  Called by FD4B.COM
%\chapter{Advanced Features con't}
%
\subsection{Phantom Commands}

It will sometimes happen, generally when vertices and loops are involved,
that one wishes that one could draw an invisible line; one
which would not actually appear in the final version of a diagram.
This is almost always the result of an alignment problem.
For instance if, for some reason, you wished to draw two photons,
parallel to one another, whose spacing was equal to a {\em gluon}
of five loops in a certain configuration.  This might be for an overlay
of a transparency.  One would like to connect the ends of the photons
with a {\em phantom} gluon.

The solution to this problem is to draw the line in {\bf phantom} mode.
Any \FEYNMAN\ commands between the statements:
\begin{verbatim}

\startphantom
     .
     .
     .
\stopphantom

\end{verbatim}
are performed but are not printed.  In particular all of the returned
parameters are evaluated as if the lines and vertices had actually been
produced.  Note also that \FEYNMAN's internal working resources are
similarly depleted.  The one item that will still be printed while 
\bs startphantom is in effect is a labelling command (or any use of \bs put).
\bs startphantom does not extend
beyond the confines of the given picture.  The following is a classic
example of the use of phantom commands in conjunction with \ddrawvertex:

% from                 VERTEST5.TEX
%  An example of using PHANTOM MODE to position lines and vertices.
%\newcount\Yone  \newcount\Xone
%\newcount\Xtwo  \newcount\Ytwo
%\newcount\Xthree  \newcount\Ythree
\begin{picture}(8000,8000)
%\THICKLINES
% This picture isn't printed out.  The \drawvertex statement sets up some
% spacing for the pic below.  It tests the `phantom mode' option.
% The \fermions below are superfluous and merely test \startphantom.
\startphantom
\drawvertex\gluon[\NE 3](0,0)[4]
\drawline\fermion[\SW\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\SE\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\E\REG](\vertextwox,\vertextwoy)[2000]
\drawline\fermion[\N\REG](\vertextwox,\vertextwoy)[2000]
\drawline\fermion[\E\REG](\vertexthreex,\vertexthreey)[2000]
\drawline\fermion[\S\REG](\vertexthreex,\vertexthreey)[2000]
\stopphantom
\global\Xtwo=\vertextwox  \global\Ytwo=\vertextwoy
\global\Xthree=\vertexthreex  \global\Ythree=\vertexthreey
\end{picture}
%\message{\vertexcount = \the\vertexcount}
\hskip 2.0in
\begin{picture}(18000,18000)
\global\Xone=\Xthree  \negate\Xone
\global\Yone=\Ytwo    \negate\Yone
\drawvertex\gluon[\NE 3](\Xone,\Yone)[4]
\drawline\fermion[\S\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\W\REG](\vertexonex,\vertexoney)[2000]
\global\Xone=\Xthree  \negate\Xone 
\global\Yone=\Ytwo    %\negate\Yone 
\drawvertex\gluon[\SE 3](\Xone,\Yone)[4]
\drawline\fermion[\W\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\N\REG](\vertexonex,\vertexoney)[2000]
\global\Xone=\Xthree  %\negate\Yone
\global\Yone=\Ytwo    \negate\Yone
%\global\advance\Xone by 30
%\global\advance \Yone by 15
\drawvertex\gluon[\NW 3](\Xone,\Yone)[4]
\drawline\fermion[\E\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\S\REG](\vertexonex,\vertexoney)[2000]
\global\Xone=\Xthree  %\negate\Xone
\global\Yone=\Ytwo    %\negate\Yone
\drawvertex\gluon[\SW 3](\Xone,\Yone)[4]
\drawline\fermion[\E\REG](\vertexonex,\vertexoney)[2000]
\drawline\fermion[\N\REG](\vertexonex,\vertexoney)[2000]
\put(-9500,-14000){The number of vertices (including phantom vertices) = 
\number\vertexcount}
\end{picture}
%\message{\vertexcount = \the\vertexcount}
