# Double square brackets in TikZ with pgfmathparse?

Why can’t I use \llbracket in the following example?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,graphs,graphs.standard,quotes}
\usepackage{stmaryrd}

\begin{document}
\begin{tikzpicture}

\def\a{{
"\llparenthesis"
,"\llbracket" % With this line, it fails!
,"\llparenthesis"
}}

\draw (0,1) node{$\llparenthesis$};
\draw (1,1) node{$\llbracket$};

\foreach \x in {0,1}
{\draw (\x, 0) node {$\pgfmathparse{\a[\x]}\pgfmathresult$};} % (*)

\end{tikzpicture}
\end{document}

1. Using \llparenthesis and \llbracket in normal draw works
2. Using \llparenthesis works when using it in \pgfmathparse, indexing from array, whereas llbracket in the same place doesn’t (line (*))
3. Both commands are from the stmaryrd package (not including the package doesn’t doesn’t allow 1.).

Activating the commented line produces this error:

! Package PGF Math Error: Unknown operator 4' or44' (in '{ "\llparenthesis " ,"\delimiter "444A471 " ,"\llparenthesis " }[0]'). See the PGF Math package documentation for explanation.

It's because one of them is a character \mathcar"444C hence "<char>" works off the bat. The other one is actually a macro expanding to \delimiter "444A471 and needs to be protected from expansion to avoid tripping up the math parser.
\def\a{{%