更改 nomecl 组标题后的垂直空间？

Question

通过该nomencl包，我将项目分成几组（通过适当定义\nomgroup）。

如何改变垂直空间后每个组标题，即组标题和该组的第一个项目之间？（我已经知道如何更改组标题前的垂直空间，如下所示\nomgroupstartsep）。

例子：

\documentclass{memoir}    
\usepackage{xstring}

\usepackage[refpage]{nomencl}
\renewcommand{\nomname}{Index of Notation}
\renewcommand*{\pagedeclaration}[1]{\unskip, #1}

\setlength{\nomitemsep}{-\parsep}

\newlength{\nomgroupstartsep}
\setlength{\nomgroupstartsep}{16pt}

\newcommand{\nomenclheader}[1]{\item[\hspace*{-\itemindent}\bfseries#1]}

\renewcommand\nomgroup[1]{%
 \itemsep\nomgroupstartsep%
  \IfStrEqCase{#1}{%
   {C}{\nomenclheader{Constants}}% 
   {S}{\nomenclheader{Sets}}%
  }%
  \itemsep\nomitemsep% restore spacing
}
\makenomenclature

\begin{document}

The number $\pi$ is the ratio of the circumference of a circle to its diameter;
$e$ is the base of the natural logarithm.%
\nomenclature[c2]{$\pi$}{ratio of circle's circumference to diameter}
\nomenclature[c1]{$e$}{base of natural logarithm}

Write $x \in A$ to mean that $x$ is an element of the set $A$.%
\nomenclature[s3]{$x \in A$}{$x$ is an element of $A$}
For sets $A$ and $B$, the notation $A \subset B$ means that $A$ is a subset of $B$.%
\nomenclature[s4]{$A \subset B$}{$A$ is a subset of $B$}    
Use lower-case letters $x$, $y$, $z$, etc., for elements of sets, and upper-case letters $A$, $B$, $C$, etc., for sets.%
\nomenclature[s1]{$x$, $y$, $z$, etc.}{elements of sets}
\nomenclature[s2]{$A$, $B$, $C$, etc.}{sets}

\printnomenclature[2.5cm]

\end{document}