What is a good survey of solutions/workarounds for 'LaTeX Error: Too many symbol fonts declared.'?

Question

We all know the dreaded error message LaTeX Error: Too many symbol fonts declared.

Is there an up to date survey of why it's there, and what can be done about it?

For example, I know that macros such as \DeclareSymbolFont gobble up some limited capacity of internally allocated counter, dating back to some magic numbers Knuth chose at the time for TeX.

I also know that newer systems (LuaTeX, XeTeX) have a higher capacity, but not for, e.g., \DeclareSymbolFont.

There are also all kinds of tricks one can do to limit the number of font families a standard package (like bbm) loads, and so on.

Is there a source that summarises the problem and solutions/workarounds comprehensively?

If there is more than one, what is the most recent/up-to-date?

Context

I often need to extract only one (or a small number) of symbols from a font. The general advice seems to be to look up that package's sourcecode and extract the bare minimum. For example, to get the \boxright symbols from the boisik font, we could use these four lines:

\DeclareFontFamily{U}{bskma}{\skewchar\font130 }
\DeclareFontShape{U}{bskma}{m}{n}{<->bskma10}{}
\DeclareSymbolFont{bskadd} {U} {bskma}{m}{n}
\DeclareMathSymbol{\boxright} {\mathbin}{bskadd} {"A0} 

The problem is that if I need to this more than a handful of times, I will start getting a Latex Error: Too many symbol fonts declared..

Partial solution

@David-carlisle 's solution below is attractive in this context, but I'm still not quite getting it to work. The basic definition is something like this:

\makeatletter
\newcommand\NewFontImport[6]{
\newcommand#1{\begingroup\mathchoice%
  {\hbox{\fontsize{\tf@size}{\tf@size}  \usefont{#2}{#3}{#4}{#5}\char#6}}%
  {\hbox{\fontsize{\tf@size}{\tf@size}  \usefont{#2}{#3}{#4}{#5}\char#6}}%
  {\hbox{\fontsize{\sf@size}{\sf@size}  \usefont{#2}{#3}{#4}{#5}\char#6}}%
  {\hbox{\fontsize{\ssf@size}{\ssf@size}\usefont{#2}{#3}{#4}{#5}\char#6}}%
  \endgroup
}}
\makeatother

This works well for, e.g., boisik \boxright:

\DeclareFontFamily{U}{bskma}{\skewchar\font130 }
\DeclareFontShape{U}{bskma}{m}{n}{<->bskma10}{}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\NewFontImport\boxright{U}{bskma}{m}{n}{"A0}

(Well, it doesn't because boisik currently has a bug with its sans serif Greek Sigma that only appears at scale mag:=0+420/600. I sent a bug report to the author. Commenting out sans serif Greek Sigma in the metafont sources works around it.)

Outstanding problems

But it doesn't seem to work across the board.

1. It doesn't work for mathabx big-operators:

\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{
      <5> <6> <7> <8> <9> <10>
      <10.95> <12> <14.4> <17.28> <20.74> <24.88>
      mathx10
      }{}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\NewFontImport\bigboxright{U}{mathx}{m}{n}{"D9}

as the \bigboxright ends up being typeset below the baseline. I think I'm passing the wrong argument to \setfont, but not sure what I should pass.

2. It doesn't work for delimiters:

\DeclareFontFamily{OMX}{MnSymbolE}{}
\DeclareFontFamily{OMX}{MnSymbolA}{}
\DeclareFontShape{OMX}{MnSymbolE}{m}{n}{
  <-6>  MnSymbolE5
  <6-7>  MnSymbolE6
  <7-8>  MnSymbolE7
  <8-9>  MnSymbolE8
  <9-10> MnSymbolE9
  <10-12> MnSymbolE10
  <12->   MnSymbolE12}{}
\DeclareFontShape{OMX}{MnSymbolE}{b}{n}{
  <-6>  MnSymbolE-Bold5
  <6-7>  MnSymbolE-Bold6
   <7-8>  MnSymbolE-Bold7
   <8-9>  MnSymbolE-Bold8
   <9-10> MnSymbolE-Bold9
  <10-12> MnSymbolE-Bold10
  <12->   MnSymbolE-Bold12}{}
\NewFontImport{\lsemsym}{OMX}{MnSymbolE}{m}{n}{'102}
\NewFontImport{\rsemsym}{OMX}{MnSymbolE}{m}{n}{'107}

I can use \lsemsym and \rsemsym on their own, but I cannot use \open\lsemsym \sum_{i=1}^n a_n \close\rsemsym as they are typeset inside a textbox, and not as variable sized delimiters. (Lots of kudos for @David Carlisle for all his patience in getting me this far!)

Answer 1

The restriction is only to 16 fonts within a single math expression, it is rather rare to need more than that. In luatex or xetex you can use \Umathchardef rather than \mathchardef to access beyond 16 families if you really need that, or even in classic TeX you can avoid the issue in the same way as \boldsymbol avoids the issue, switch to an hbox then set up a different font regime just for the single character then return to the outer formula.

This is a single math expression set with pdflatex with 24 different Z

\documentclass{article}

\usepackage{amsmath,amsfonts}
\DeclareFontEncoding{LS1}{}{}
\DeclareFontEncoding{LS2}{}{\noaccents@}
\DeclareFontSubstitution{LS1}{stix2}{m}{n}
\DeclareFontSubstitution{LS2}{stix2}{m}{n}

\begin{document}

$
Z                                       %  0
\mathrm{Z}                              %  1
\mathit{Z}                              %  2
\mathsf{Z}                              %  3
\mathbf{Z}                              %  4
\mathfrak{Z}                            %  5
\mathcal{Z}                             %  6
\mathbb{Z}                              %  7
\mathtt{Z}                              %  8
\hbox{\usefont{OML}{txmi}{m}{it}Z}      %  9
\hbox{\usefont{OML}{txmi}{bx}{it} Z}    % 10
\hbox{\usefont{OT1}{txr}{m}{n} Z}       % 11
\hbox{\usefont{OT1}{txr}{bx}{n} Z}      % 12
\hbox{\usefont{LS1}{stix2}{m}{n} Z}     % 13
\hbox{\usefont{LS1}{stix2}{m}{it} Z}    % 14
\hbox{\usefont{LS1}{stix2}{b}{n} Z}     % 15
\hbox{\usefont{LS2}{stix2}{b}{it} Z}    % 16
\hbox{\usefont{LS2}{stix2}{m}{n} Z}     % 17
\hbox{\usefont{LS2}{stix2}{m}{it} Z}    % 18
\hbox{\usefont{LS2}{stix2}{b}{n} Z}     % 19
\hbox{\usefont{LS2}{stix2}{b}{it} Z}    % 20
\hbox{\usefont{LS1}{stix2frak}{m}{n} Z} % 21
\hbox{\usefont{LS1}{stix2frak}{b}{n} Z} % 22
\hbox{\usefont{LS2}{stix2cal}{m}{n} Z}  % 23
$
\end{document}

Answer 2

I can explain the "outstanding problem #1".

The \bigboxright symbol is declared in mathabx as

\DeclareMathSymbol{\bigboxright}     {1}{mathx}{"D9}

Note that it has category 1, and so it is operator. TeX has special function for displaying operators (see make_op in the Knuth's tex.web file). In particular, it can switch to a larger version of the symbol in display mode and shift the symbol vertically if necessary. To emulate this behaviour, I suggest the following modification of the code:

\newcommand\makeopNewFont[6]{
  \setbox0\hbox{\fontsize{#1}{#1}\usefont{#2}{#3}{#4}{#5}\char#6}%
  \lower\dimexpr .5\ht0-.5\dp0-\fontdimen22\textfont2\copy0
}
\newcommand\NewFontImportOp[7]{
  \newcommand#1{%
    \mathop{\mathchoice%
        {\makeopNewFont{\tf@size}{#2}{#3}{#4}{#5}{#7}}%
        {\makeopNewFont{\tf@size}{#2}{#3}{#4}{#5}{#6}}%
        {\makeopNewFont{\sf@size}{#2}{#3}{#4}{#5}{#6}}%
        {\makeopNewFont{\ssf@size}{#2}{#3}{#4}{#5}{#6}}%
    }%
  }%
}
%....
\NewFontImportOp\bigboxright{U}{mathx}{m}{n}{"D9}{"F9}

Unfortunately, I don't know an easy way to determine the code corresponding to the paired (a larger) symbol ("F9 in this case), because of this pairing is stored inside of the font (so-called charlist entries). To found "F9, I've analyzed \tracingoutput-generated log of a test page with original \bigboxright command. Alternative way is to look at all glyphs using the fonttable package and found a match there.

Most likely it is possible to emulate most of other math categories as well (by just prepending \hbox with \mathbin, \mathrel, etc. to get correct kerning):

\newcommand\NewFontImport[7]{%
  \newcommand#1{%
    #5{\mathchoice%
        {\hbox{\fontsize{\tf@size}{\tf@size}\usefont{#2}{#6}{#3}{#4}\char#7}}%
        {\hbox{\fontsize{\tf@size}{\tf@size}\usefont{#2}{#6}{#3}{#4}\char#7}}%
        {\hbox{\fontsize{\sf@size}{\sf@size}\usefont{#2}{#6}{#3}{#4}\char#7}}%
        {\hbox{\fontsize{\ssf@size}{\ssf@size}\usefont{#2}{#6}{#3}{#4}\char#7}}%
    }%
  }%
}

% Note: I've changed the order of arguments to match \DeclareMathSymbol
\NewFontImport\times{U}{m}{n}{\mathbin}{matha}{"02} 

But emulation of delimiters is too much complicated (because of construction of left/right delimiters of arbitrary size). It's where I give up.