\documentclass{article}
\usepackage{glossaries}
\usepackage{mathtools}
\usepackage{xparse}

% Custom glossary commands 1
\glsaddkey
  {maccusative}          % key
  {}                     % default value
  {\glsentrymaccusative} % no link cs
  {\Glsentrymaccusative} % no link ucfirst cs
  {\glsmacc}             % link cs
  {\Glsmacc}             % link ucfirst cs
  {\GLSmacc}             % link all caps cs

\DeclareDocumentCommand{\testddc}{ o m }{%
first #1 second #2
}

% Custom glossary commands 2
\glsaddkey{plural}{}
  {\glsentryplural}
  {\Glsentryplural}
  {\glspl}
  {\Glspl}
  {\GLSpl}

\let\testlet=\leftrightarrow

\newcommand{\testnewcommand}{Hello world}

\DeclarePairedDelimiter\testpaireddelim{\lvert}{\rvert}

\begin{document}

\end{document}
