\documentclass{article}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}

%%%%%%%%%% Start TeXmacs macros
\newcommand{\dueto}[1]{\textup{\textbf{(#1) }}}
\newcommand{\tmtextit}[1]{{\itshape{#1}}}
\newtheorem{lemma}{Lemma}
%%%%%%%%%% End TeXmacs macros


\newcommand{\dueto}[1]{\tmtextit{(#1) }}

\begin{document}



\begin{lemma}
  {\dueto{de Benito}}All foos are bars.
\end{lemma}

\end{document}
