Paired delimiter made of several brackets

Question

I want to define a stretchable delimiter made of several brackets, so that they are stretched to the same height, i. e. if I declare

\DeclarePairedDelimiter{\double}{((}{))}

then \double{...} should make the outer () the same size as the inner, like

the code for this fraction is

\delimitershortfall=0pt

\[
    \left(\left( \dfrac{x}{y}\right)\right)
\]

But right now I get some weird behaviour:

How can I fix it?

Full MWE

\documentclass{article}

\usepackage{mathtools}

\usepackage{unicode-math}

\setmathfont{STIX Two Math}

\DeclarePairedDelimiter{\double}{((}{))}

\DeclarePairedDelimiter{\triple}{\{[(}{>)]}

\DeclarePairedDelimiter{\dif}{(}{))}


\begin{document}

    \begin{align}
        \double{x}\\
        \double*{\dfrac{n}{k}}\\
        \triple{x}\\
        \triple*{\dfrac{a}{b}}\\
        \dif*{\dfrac{x}{y}}
    \end{align}

\end{document}

Answer 1

The size command is only applied to the first delimiter, not to the following ones; there's no error message, but what you get is a byproduct of the actual implementation.

You can use \DeclarePairedDelimiterX:

\documentclass{article}

\usepackage{mathtools}
\usepackage{unicode-math}

\setmathfont{STIX Two Math}

\DeclarePairedDelimiterX{\double}[1]{(}{)}{\delimsize(#1\delimsize)}
\DeclarePairedDelimiterX{\triple}[1]{\{}{]}{%
  \delimsize[\delimsize(#1\delimsize\rangle\delimsize)%
}
\DeclarePairedDelimiterX{\dif}[1]{(}{)}{#1\delimsize)}

\begin{document}

\begin{align}
\double{x}\\
\double*{\dfrac{n}{k}}\\
\triple{x}\\
\triple*{\dfrac{a}{b}}\\
\dif*{\dfrac{x}{y}}
\end{align}

\end{document}

A full reimplementation, but the numbers of delimiters must match, use . for balancing.

\documentclass{article}

\usepackage{mathtools,mleftright}
\usepackage{unicode-math}

\setmathfont{STIX Two Math}

\ExplSyntaxOn
\NewDocumentCommand{\DeclareMultiPairedDelimiter}{mmm}
 {
  \NewDocumentCommand{#1}{sO{}m}
   {
    \IfBooleanTF{##1}
     { \fraiman_multi_open_lr:n { #2 } }
     { \fraiman_multi_open:nn { ##2 } { #2 } }
    ##3
    \IfBooleanTF{##1}
     { \fraiman_multi_close_lr:n { #3 } }
     { \fraiman_multi_close:nn { ##2 }{ #3 } }
   }
 }

\cs_new_protected:Nn \fraiman_multi_open_lr:n
 {
  \tl_map_inline:nn { #1 } { \mleft ##1 }
 }
\cs_new_protected:Nn \fraiman_multi_close_lr:n
 {
  \tl_map_inline:nn { #1 } { \mright ##1 }
 }
\cs_new_protected:Nn \fraiman_multi_open:nn
 {
  \mathopen { \tl_map_inline:nn { #2 } {\tl_if_eq:nnF { ##1 } { . } { #1 ##1 } } }
 }
\cs_new_protected:Nn \fraiman_multi_close:nn
 {
  \mathclose { \tl_map_inline:nn { #2 } {\tl_if_eq:nnF { ##1 } { . } { #1 ##1 } } }
 }
\ExplSyntaxOff

\DeclareMultiPairedDelimiter{\double}{((}{))}
\DeclareMultiPairedDelimiter{\triple}{\{[(}{>)]}
\DeclareMultiPairedDelimiter{\dif}{(.}{))}

\begin{document}

\begin{align}
\double{x}\\
\double*{\dfrac{n}{k}}\\
\triple{x}\\
\triple*{\dfrac{a}{b}}\\
\dif*{\dfrac{x}{y}}
\end{align}

\end{document}

Answer 2

\usepackage{xparse,mleftright}

\ExplSyntaxOn

\NewDocumentCommand \double { s o m }
 {
  \IfBooleanTF {#1}
   { \mleft(\mleft( #3 \mright)\mright) }
   {
    \IfValueTF {#2}
     {
      \use:c { \cs_to_str:N #2 l } (
      \use:c { \cs_to_str:N #2 l } (
      #3
      \use:c { \cs_to_str:N #2 r } )
      \use:c { \cs_to_str:N #2 r } )
     }
     { (( #3 )) }
   }
 }

\ExplSyntaxOff