src/ZF/Induct/document/root.tex

 \documentclass[11pt,a4paper]{article}
 \usepackage{isabelle,isabellesym}
+\usepackage{amssymb}
 \usepackage[utf8]{inputenc}
 \usepackage{pdfsetup}
 
 \urlstyle{rm}
 \isabellestyle{it}