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%\chapter{Advanced Features con't :  REVIEW EXER FOR CH 4.}
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% links:  lines,vertices
% stems:  lines,vertices
% caps:   lines,vertices
% loops
% saving and re-using:  \Xone, \newcount etc.; \drawandsave, \savebox etc.
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\section{Review Exercises for Chapter 4}

{\Large\bf A:}
\vskip 0.2in
Describe (but don't reproduce) how you would make the following
example of `Feyn Art':

% THEORY.TEX 
%\vskip 1in
\vskip 0.5in
\hskip 0.75in
\begin{picture}(28000,8000)
%T:
\drawline\gluon[\N\CENTRAL](0,0)[5]
\global\Yone=\gluonbacky
\drawline\fermion[\W\REG](\gluonbackx,\gluonbacky)[4000]
\drawline\fermion[\E\REG](\gluonbackx,\gluonbacky)[4000]
\global\advance\pbackx by 1000
%H:
\drawline\fermion[\N\REG](\pbackx,\gluonfronty)[\gluonlengthy]
\drawline\photon[\E\REG](\pmidx,\pmidy)[5]
\drawline\fermion[\N\REG](\pbackx,\gluonfronty)[\gluonlengthy]
\global\advance\pbackx by 1000
%E:
\drawline\fermion[\N\REG](\pbackx,\gluonfronty)[\gluonlengthy]
\global\advance\pmidy by -300
\drawline\gluon[\E\FLIPPEDCURLY](\pmidx,\pmidy)[4]
\global\advance\plengthx by 500
\drawline\fermion[\E\REG](\gluonfrontx,0)[\plengthx]
\drawline\fermion[\E\REG](\gluonfrontx,\Yone)[\plengthx]
\global\advance\pbackx by 4000
%O:
\drawloop\gluon[\NW 8](\pbackx,300)
\global\advance\loopbackx by 5500
%R:
\drawline\gluon[\S\CURLY](\loopbackx,\Yone)[3]
\global\Xone=\boxlengthy  \double\Xone  \multroothalf\Xone
\put(\pmidx,\pmidy){\oval(\Xone,\boxlengthy)[r]}
\drawline\fermion[\S\REG](\pbackx,\pbacky)[\pbacky]
\global\advance\pfrontx by 500
%\global\advance\pfronty by 30
\global\advance\pfronty by 20
\drawline\photon[\SE\CURLY](\pfrontx,\pfronty)[4]
%Y:
\global\advance\pbackx by 2600
\drawline\photon[\N\CURLY](\pbackx,0)[3]
\global\Ytwo=\Yone
\negate\plengthy
\global\advance\Ytwo by \plengthy
\double\Ytwo  \multroothalf\Ytwo
\drawline\fermion[\NE\REG](\photonbackx,\photonbacky)[\Ytwo]
\drawline\fermion[\NW\REG](\photonbackx,\photonbacky)[\Ytwo]
\end{picture}
\vskip 0.57in
{\noindent{\Large\bf B:}}
\vskip 0.2in
Reproduce the `balloon in the tree' diagram:
\vskip 0.2in

\begin{picture}(28000,28000)
\THICKLINES
%set up position of bottom of loop
\startphantom
\drawloop\gluon[\NE 0](0,0)
\stopphantom
\global\Xone=\loopfrontx  % diameter of `true' loop
\drawloop\gluon[\S 7](12000,18000)
\global\Xtwo=\gluonbackx
\global\Ytwo=\gluonbacky
% draw gluon vertex
\global\advance\loopmidy by \Xone
\global\stemlength=400
\stemvertex1\drawvertex\gluon[\S 3](\loopmidx,\loopmidy)[3]
% Determine diameter and centre of fermion loop
\negate\Xtwo
\global\advance\Xtwo by \loopfrontx
\global\Xthree=\Xtwo %  Store.  Will use shortly
\double\Xtwo  % \Xtwo is now the diameter of the fermion loop
\put(\loopfrontx,\Ytwo){\circle{\Xtwo}}
% Lastly the photon
% This begins located at root 1/2 times radius in x & y direction from centre
\multroothalf\Xthree %  This is why we stored it.
\global\advance\loopfrontx by \Xthree
\global\advance\Ytwo by \Xthree
\drawline\photon[\NE\FLIPPED](\loopfrontx,\Ytwo)[5]
\end{picture}