I wanted my "see" cross-references within the index to be formatted in the form:
this (see that)
So thanks to Change index "see also" and "see" format, I use the definitions of \seeonly shown in the following source file.
But I also want the page headings of the index to show the first and last index entry on each page there. So I also defined \idxmark, use the index page style, and employ the index .ist style file described in the memoir documentation and now also included in my source.
% File indexdoc.tex
\documentclass{memoir}
\newcommand{\idxmark}[1]{#1\markboth{#1}{#1}}
\makepagestyle{index}
\makeheadrule{index}{\textwidth}{\normalrulethickness}
\makeevenhead{index}{\rightmark}{}{\leftmark}
\makeoddhead{index}{\rightmark}{}{\leftmark}
\makeevenfoot{index}{}{\thepage}{}
\makeoddfoot{index}{}{\thepage}{}
\newcommand\gobbleone[1]{}
\newcommand{\seeonly}[2]{\, (\emph{\seename} #1)}
\let\oldindex\index
\renewcommand{\index}[1]{\def\exptoindex{#1}\expandafter\oldindex\expandafter{\exptoindex}}
\newcommand{\indexsee}[2]{\index{#1@#1\protect\gobbleone|seeonly{#2}}}
\begin{filecontents}{indexdoc.ist}
% MakeIndex style file
% output main entry <entry> as: \item \idxmark{<entry>},
item_0 "\n\\item \\idxmark{"
delim_0 "}, "
% not forgetting the subitem case
item_x1 "} \n \\subitem "
\end{filecontents}
\makeindex
\begin{document}
This is some text\indexsee{text}{verbiage}. Which is a synonym for verbiage\index{verbiage}.
\backmatter
\pagestyle{index}
\printindex
\end{document}
Those two things, alas, clash; I get error message:
Writing index file indexdoc.idx
(./indexdoc.aux) [1{/Users/user/Library/texlive/2016/texmf-var/fonts/map/
pdftex/updmap/pdftex.map}] [2] (./indexdoc.ind
./indexdoc.ind:3: Argument of \gobbleone has an exra }.Runaway argument?
./indexdoc.ind:3: Paragraph ended before \gobbleone was complete.
<to be read again>
\par
l.3 \item \idxmark{text\gobbleone }
, \seeonly{verbiage}{1}
?
<inserted text>
\par
l.3 \item \idxmark{text\gobbleone }
, \seeonly{verbiage}{1}
?
Without the \idxmark definition, the index page style, and the index .ist style file from memman.pdf, the definition of \seeonly (in terms of \gobble) causes no problem.
How can the clash be resolved?
Originally accepted answer not robust with math
I had originally accepted "siracusa"'s answer, which redefined \idxmark. However, I now find that it fails the \indexsee command — but is OK for the \indexAlso command — when the first argument has the form
alfatxt@$alfamath$
(for alphabetizing math expression properly in the index).
% File indexdoc2.tex
\documentclass{memoir}
\newcommand{\idxmark}[1]{{\let\gobbleone\relax\markboth{#1}{#1}}#1}
\makepagestyle{index}
\makeheadrule{index}{\textwidth}{\normalrulethickness}
\makeevenhead{index}{\rightmark}{}{\leftmark}
\makeoddhead{index}{\rightmark}{}{\leftmark}
\makeevenfoot{index}{}{\thepage}{}
\makeoddfoot{index}{}{\thepage}{}
\let\oldindex\index
\renewcommand{\index}[1]{\def\exptoindex{#1}\expandafter\oldindex\expandafter{\exptoindex}}
\newcommand\gobbleone[1]{}
\newcommand{\seeonly}[2]{\, (\emph{\seename} #1)}
\newcommand{\indexsee}[2]{\index{#1@#1\protect\gobbleone|seeonly{#2}}}
\newcommand{\Also}[2]{\unskip\emph{See also} #1}
\newcommand{\indexAlso}[2]{\index{#1!zzzz@\protect\gobbleone|Also{#2}}}
\begin{filecontents}{indexdoc2.ist}
% MakeIndex style file
% output main entry <entry> as: \item \idxmark{<entry>},
item_0 "\n\\item \\idxmark{"
delim_0 "}, "
% not forgetting the subitem case
item_x1 "} \n \\subitem "
\end{filecontents}
\makeindex
\begin{document}
This is some text\indexsee{text}{verbiage}. Which is a synonym for verbiage\index{verbiage}.
$\pi$\index{pi@$\pi$ (math constant)} % OK
$\rho$ \indexsee{$\rho$}{density} % OK
$\sigma$ \indexsee{sigma@$\sigma$}{conductivity} % NO entry in index!
$\mu$ \indexAlso{$\mu$}{magnetic moment} % OK
$\alpha$ \indexAlso{alpha@$\alpha$}{angular acceleration} % OK !
\backmatter
\pagestyle{index}
\printindex
\end{document}


\seeonlywhen its 1st argument has formtxt@$math$: no index entry appears in that case. – murray Oct 09 '16 at 00:27