rebber-plugins: missing some escapement

[artragis]

Hello,

I've found this expression which is badly sanitized :

input

$$\nabla^2 E(\vec x) + [n(\vec{x})\,k]^2\,E(\vec x) = -s(\vec x),$$

Current output

$$E(\vec x) + [n(\vec{x}) ]^2 (\vec x) = -s(\vec x),$$

Expected output

$$\nabla^2 E(\vec x) + [n(\vec{x})\,k]^2\,E(\vec x) = -s(\vec x),$$

Explanation

It seems that we do not have \nabla in the latex constants file even if it is properly supported by katex.

Then the other issues seems to come from the \,. It is a valid latex, not errored by KaTeX but removed by zmd.