Not parallel symbols using slashes

Question

In France, we write d_1 // d_2 to indicate that the lines d_1 and d_2 are parallel.

I would like to have a "not parallel" symbol. What is the best way to do that ?

Advices alone are welcome.

\documentclass[12pt]{article}

\let\stdparallel\parallel
\renewcommand\parallel{\mathbin{/\!/}}

\begin{document}

$d_1 \parallel d_2$

\end{document}

Answer 1

I would keep the slashes tighter; for the negation, the specular image of \smallsetminus seems good.

\documentclass{article}
\usepackage{amsmath,amssymb,graphicx}

\renewcommand{\parallel}{\mathrel{/\mkern-5mu/}}
\makeatletter
\newcommand{\notparallel}{%
  \mathrel{\mathpalette\not@parallel\relax}%
}
\newcommand{\not@parallel}[2]{%
  \ooalign{\reflectbox{$\m@th#1\smallsetminus$}\cr\hfil$\m@th#1\parallel$\cr}%
}
\makeatother

\begin{document}

$a \parallel b \notparallel c$

\end{document}