Intersection of double lines in tabular environment, change which pair breaks and which doesn't

Question

Let's consider a very simple table:

\begin{tabular}{c || c | c}
 & x & y \\ \hline \hline
 a & 1 & 2 \\ \hline
 b & 3 & 4
\end{tabular}

This produces the following table:

Notice that at the intersection of the double lines (top left), the vertical lines break and the horizontal lines don't. Is there a way to flip this behavior, i.e. make the horizontal lines break while the vertical lines go through?

Answer 1

It's not difficult with hhline.

\documentclass{article}
\usepackage{hhline}

\begin{document}

\begin{tabular}{c || c | c}
 & x & y \\
\hhline{=||=|=}
 a & 1 & 2 \\ \hline
 b & 3 & 4
\end{tabular}

\end{document}

But the result isn't pretty at all. Compare.

\documentclass{article}
\usepackage{hhline}
\usepackage{booktabs}% for the second table

\begin{document}

\begin{tabular}{c || c | c}
 & x & y \\ 
\hhline{=||=|=}
 a & 1 & 2 \\ \hline
 b & 3 & 4
\end{tabular}
\qquad
\begin{tabular}{@{}ccc@{}}
\toprule
 & x & y \\ 
\midrule
 a & 1 & 2 \\
 b & 3 & 4 \\
\bottomrule
\end{tabular}

\end{document}