Labelling Theorem Environments across documents

Question

This might seem like a really obvious question to some of you but I am having a bit of trouble.

I use Theorem (and Lemma etc.) environments in my documents and reference them in the proof of others - pretty standard.

The trouble I am having is what to call my theorems - the ones without names anyway.

An obvious way is to number them the same way they appear in the document, but if I then go back and add or remove theorem environments this all goes kablooey.

Does anyone have a nice logical system for doing this?

Minimal Working Example

\documentclass{article}

\newtheorem{thm}{Theorem}
\newtheorem{prop}[thm]{Proposition}
\newtheorem{lem}[thm]{Lemma}


\begin{document}

\begin{lem}\label{lem2}
    This is the 1st lemma (2nd 'theorem') I write, but it clarifies the proof of the theorem below, so I place it above.
\end{lem}
\begin{proof}
    The proof of lemma.
\end{proof}

\begin{thm}\label{thm1}
    This is the 1st theorem I write.
\end{thm}
\begin{proof}
    The proof of theorem using Lemma \ref{lem2}.
\end{proof}
\end{document}

Answer 1

If I understand it correctly, your problem is that you have some \labels in some document and want to refer to them using \ref in a separate (as in "compiled separately") document.

This is a job for the xr package:

If one document needs to refer to sections of another, say aaa.tex, then this package may be loaded in the main file, and the command \externaldocument{aaa} given in the preamble. Then you may use \ref and \pageref to refer to anything which has been given a \label in either aaa.tex or the main document. You may declare any number of such external documents.

Answer 2

Try naming the theorems as per convenience and then label them using preassigned suitable names. For example, \label{Continuity}, \label{date11.1.19} etx