adding missing pieces

tikz-feynman/gm2-dirac.tex

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+\documentclass[11pt]{standalone}
+% \usepackage{amssymb} %maths
+% \usepackage{amsmath} %maths
+% \usepackage[utf8] %useful to type directly diacritic characters
+\usepackage{tikz-feynman} 
+% \tikzfeynmanset{compat=1.0.0}
+
+\begin{document}
+
+\feynmandiagram [vertical=a to b] {
+  % a [crossed dot] -- [photon, thick, edge label=\(\gamma\)] b [dot],
+  a -- [photon, thick, edge label=\(\gamma\)] b,
+  i1 [particle=\(\mu\)] -- [fermion, thick] b -- [fermion, thick] f1 [particle=\(\mu\)],
+  % f1 -- [fermion] b -- [fermion] f2,
+};
+
+% \feynmandiagram [horizontal=a to b] {
+  % i1 [particle=\(e^{-}\)] -- [fermion] a -- [fermion] i2 [particle=\(e^{+}\)],
+  % a -- [photon, edge label=\(\gamma\), momentum'=\(k\)] b,
+  % f1 [particle=\(\mu^{+}\)] -- [fermion] b -- [fermion] f2 [particle=\(\mu^{-}\)],
+% };
+  
+\end{document}