fix ret

main.tex

@@ -18,6 +18,7 @@
 \newcommand{\M}{\mathcal{M}}
 \newcommand{\sq}{\square}
 \newcommand{\lett}{\text{let}\,\,}
+\newcommand{\ret}{\text{ret}\,\,}
 
 \begin{document}
 \title{Call-by-value Gradual Type Theory}