You know, in latex you can type
\neq
to make an inequation symbol:
`$latex \neq$`
Of course, you don’t want to remember how to type in latex all symbols with backslash on them. So here is a general way to add backslash on any symbol in latex: add `\not` in front of a normal symbol.
Hence, `\not\rightsquigarrow` becomes $latex \not\rightsquigarrow$. `\not\subseteq` becomes $latex \not\subseteq$. so on so forth.