When I use the proof environment, the output does not show the tombstone informs3 class.
The class can be download here by selecting INFORMS Journal on Computing Style Files.
I tired the recommendations posted here and here. If I use a regular document class, it works, but it does not properly work in informs3 class.
Here is a MWE.
\documentclass[ijoc,nonblindrev]{informs3}
\usepackage{amsmath}
\usepackage[amsmath,amsthm,thmmarks]{ntheorem}
\begin{document}
\begin{proof}
here is the proof.
\end{proof}
\end{document}
I also tried to add the symbol manually, but still did not work
\usepackage{amsmath,amsthm}
\makeatletter
\newcommand{\altqedhere}{%
\ifmeasuring@\else\sbox0{\popQED}\fi
\tag*{\qedsymbol}%
}
\makeatother

informs3based issue, I really don't know where to look for the answer. – whitepanda Dec 31 '19 at 03:43informs3.clsfile and searched for aproofenvironment. Luckily I found quickly the definition! My answer here is just a slight modification of the definition of\endproofin the source file. It's good to keep in mind that whatever LaTeX does, it's coded somewhere in some source files, so all of the information is always coded somewhere. Sometimes it can be hard to read, but it can also be a good way to learn! – Vincent Dec 31 '19 at 04:03