Custom Environment Numbering

Question

I am trying to achieve a goal extremely similar to that requested in Custom theorem numbering. There egreg explains how to create a custom theorem environment. However, one must specify whether it is to be a Theorem, Proposition or whatever. He uses the following code.

\usepackage{amsthm}
\newtheorem{innercustomgeneric}{\customgenericname}
\providecommand{\customgenericname}{}
\newcommand{\newcustomtheorem}[2]{%
  \newenvironment{#1}[1]
  {%
   \renewcommand\customgenericname{#2}%
   \renewcommand\theinnercustomgeneric{##1}%
   \innercustomgeneric
  }
  {\endinnercustomgeneric}
}
\newcustomtheorem{customthm}{Theorem}
\newcustomtheorem{customlemma}{Lemma}

I would not like to make this final decision, but rather have an environment customgen which when called with title uses that title. Consider the following, using egreg's command.

\begin{customthm}{CUSTOM_LABEL}
    This is Theorem CUSTOM_LABEL
\end{customthm}

This creates an environment which starts Theorem CUSTOM_LABEL. I would like

\begin{customgen}{CUSTOM_TITLE}
   This may or may not be a theorem
\end{customgen}

to start CUSTOM_TITLE.

I can nearly achieve this by replacing the [2] with [1] and commenting out the command \renewcommand\customgenericname{#2}. However, there is extra whitespace on the left. I don't know how to get rid of that.

Answer 1

It's much simpler than that.

\documentclass{article}
\usepackage{amsthm}
\newtheorem{customthminner}{\customthmname}[section]
\newcommand{\customthmname}{}
\newenvironment{customthm}[1]
 {\renewcommand{\customthmname}{#1}\customthminner}
 {\endcustomthminner}
\begin{document}
\section{Famous theorems}
\begin{customthm}{Gauss's lemma}
Blah blah
\end{customthm}
\begin{customthm}{Fermat's last theorem}
Blah blah
\end{customthm}
\end{document}

If you don't want numbering, use \newtheorem* (and no optional argument).