Code:
\documentclass[a4paper]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{amsmath,amssymb,nomencl,ifthen}
\makenomenclature
\newcommand{\nomaltname}{Symbole}
\newcommand{\nomaltpreamble}{}
\newcommand{\nomaltpostamble}{}
\newcommand{\usetwonomenclatures}{\nomenclature[\switchnomitem]{}{}}
\newcommand{\switchnomitem}{R}
\renewcommand{\nomgroup}[1]{%
\ifthenelse{\equal{#1}{\switchnomitem}}{\switchnomalt}{}}
\newcommand{\switchnomalt}{%
\end{thenomenclature}
\renewcommand{\nomname}{\nomaltname}
\renewcommand{\nompreamble}{\nomaltpreamble}
\renewcommand{\nompostamble}{\nomaltpostamble}
\begin{thenomenclature}
}
\begin{document}
Ein MB\nomenclature{MB}{Minimalbeispiel} ist's gerade nicht geworden\dots
ein Symbol \nomenclature[s]{$\eta$}{Wirkungsgrad}\dots
\usetwonomenclatures
\printnomenclature
\end{document}
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