Symbols for logical expressions

[TanTrisGit]

Hey, I could use some more logical expression symbols.

• \models, but for equivalence. In some cases, \equiv is used, but not in general. There is the \models symbol, but only from left to right. Here, logical equivalence is denoted by the \models symbol followed by the \models symbol mirrored along the y-axis. (EDIT: edemaine suggested supporting "reflectbox")
• \sslash (from stmaryrd package). A double slash for function declarations in first order logic. The regular slashes are too far apart from each other to be readable in this context. (EDIT: edemaine suggested using a macro and negative \hspace, which works for me)

These two would be awesome. I am sure I will find more over time, but I am missing especially the first one pretty much daily. Thank you for the great work!

[edemaine]

A backwards \models could be obtained if we supported \reflectbox (via \mathop{\reflectbox{\models}}), which I feel like might be relatively easy to do via CSS transform, and quite broadly useful.

stmaryrd seems to implement \sslash as a custom font symbol, but we could also consider a simple implementation like /\hspace{-0.25em}/ which looks like this:
. You could do this now via custom macros.