New symbol derived from [a, b]

Question

I would like to draw the following new symbol, so that I can use it in sentences.

Which tool would you advise? TikZ?
Thanks

campa · Accepted Answer

Before someone comes with a TikZ solution...
documentclass{article}

% usepackage{amsmath}

makeatletter
newcommand*{foo}{mathpalette@foo}
newcommand*{@foo}[2]{%
   % rule thickness
   @tempdima=fontdimen8
       ifx#1displaystyletextfontelse
       ifx#1textstyletextfontelse
       ifx#1scriptstylescriptfontelse
       scriptscriptfontfififi 3
   % spacing around the symbols 0.5ex
   @tempdimb=0.5fontdimen5
       ifx#1displaystyletextfontelse
       ifx#1textstyletextfontelse
       ifx#1scriptstylescriptfontelse
       scriptscriptfontfififi 2
   sbox0{m@thkern@tempdimb$#1#2$kern@tempdimb}%
   ht0=dimexprht0+@tempdimbrelax
   dp0=dimexprdp0+@tempdimbrelax
   lowerdimexprdp0+@tempdimarelax
      hbox{%
        vrule@width@tempdima
        vbox{offinterlineskip
          hb@xt@wd0{vrule@height@tempdima@width@tempdimbhfilvrule@height@tempdima@width@tempdimb}%
          box0
          hrule@height@tempdima
        }%
        vrule@width@tempdima
      }%
}
makeatother

begin{document}

$foo{a,b}$
$scriptstylefoo{a,b}$
$scriptscriptstylefoo{a,b}$

medskip
$displaystylefoo{-frac{a}{2},frac{a}{2}}$
$foo{-frac{a}{2},frac{a}{2}}$
$scriptstylefoo{-frac{a}{2},frac{a}{2}}$
$scriptscriptstylefoo{-frac{a}{2},frac{a}{2}}$

end{document}

The two lengths scale accordingly to the current math style: @tempdima is the standard rule thickness, while @tempdimb is basicall half ex (the latter could be fixed without resorting to fontdimen).

If you uncomment amsmath then the result will differ slightly in scriptscriptstyle, where you get
without amsmath:

with amsmath:

egreg · Answer

If your symbols don't have excessive depth (no fractions, for instance), you can get away with
documentclass{article}
usepackage{amsmath,array}

makeatletter
newcommand{segment}[1]{%
  biglceilcolas@segment{#1}bigrceil
}
newcommand{colas@segment}[1]{%
  mspace{-4.85mu}%
  underline{mspace{5mu}#1vphantom{j}mspace{5mu}}%
  mspace{-4.85mu}%
}
makeatother

begin{document}
Huge

indentrlap{vrule height0pt depth0.1pt width 4cm}%
$segment{a,b}={ta+(1-t)b: tin[0,1]}$

indentrlap{vrule height0pt depth0.1pt width 4cm}%
$segment{p,q}={tp+(1-t)q: tin[0,1]}$

end{document}

New symbol derived from [a, b]

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