Symbol for anamorphism

The paper uses square brackets and parentheses, with a rather poor result.

Here's a better one.

\documentclass{article}
\usepackage{amsmath}
\usepackage{pict2e}

\newcommand{\lbparen}{%
  \mathopen{%
    \sbox0{$()$}%
    \setlength{\unitlength}{\dimexpr\ht0+\dp0}%
    \raisebox{-\dp0}{%
      \begin{picture}(.32,1)
      \linethickness{\fontdimen8\textfont3}
      \roundcap
      \put(0,0){\raisebox{\depth}{$($}}
      \polyline(0.32,0)(0,0)(0,1)(0.32,1)
      \end{picture}%
    }%
  }%
}

\newcommand{\rbparen}{%
  \mathclose{%
    \sbox0{$()$}%
    \setlength{\unitlength}{\dimexpr\ht0+\dp0}%
    \raisebox{-\dp0}{%
      \begin{picture}(.32,1)
      \linethickness{\fontdimen8\textfont3}
      \roundcap
      \put(-0.08,0){\raisebox{\depth}{$)$}}
      \polyline(0,0)(0.32,0)(0.32,1)(0,1)
      \end{picture}%
    }%
  }%
}

\begin{document}

$\lbparen g,p\rbparen$

\end{document}

If you need to use the Euler fonts, some values need to be changed.

\documentclass{article}
\usepackage{amsmath}
\usepackage{eulervm}
\usepackage{pict2e}

\newcommand{\lbparen}{%
  \mathopen{%
    \sbox0{$()$}%
    \setlength{\unitlength}{\dimexpr\ht0+\dp0}%
    \raisebox{-\dp0}{%
      \begin{picture}(.3,1)
      \linethickness{\fontdimen8\textfont3}
      \roundcap
      \put(0,0){\raisebox{\depth}{$($}}
      \polyline(0.3,0.07)(0,0.07)(0,0.98)(0.3,0.98)
      \end{picture}%
    }%
  }%
}

\newcommand{\rbparen}{%
  \mathclose{%
    \sbox0{$()$}%
    \setlength{\unitlength}{\dimexpr\ht0+\dp0}%
    \raisebox{-\dp0}{%
      \begin{picture}(.3,1)
      \linethickness{\fontdimen8\textfont3}
      \roundcap
      \put(-0.08,0){\raisebox{\depth}{$)$}}
      \polyline(0,0.07)(0.3,0.07)(0.3,0.98)(0,0.98)
      \end{picture}%
    }%
  }%
}

\begin{document}

$\lbparen g,p\rbparen$

\end{document}

The solution which does not need LaTeX picture environment is here. Only \vrule and \hrule primitives are used. But this solution does not care about script and scriptscript sizes. But egreg's solution does not care about it too.

\def\lbparen{\mathopen{\vrule \lower.249em\vbox to1em{%
   \hrule height.2pt width.3em\vss \hrule height.2pt}%
   \kern-.32em(}}
\def\rbparen{\mathclose{)\kern-.32em\lower.249em\vbox to1em{%
   \hrule height.2pt width.3em\vss \hrule height.2pt}%
   \vrule}}

$\lbparen g, p\rbparen$

\bye