a in b can be written as a ∈ b with the mathematical symbol ∈. And the opposite of that, ∉ is available as an operator too, so you can do a ∉ b for !(a in b).

∈ can be typed with \in<TAB>, ∉ with \notin<TAB> in the REPL.

It would be helpful if all such operators were “aliased” to normal words that one could type on a keyboard, however. I don’t know about you, but I don’t have a ∉ key on mine

What we need, I feel, is a way to create infix aliases or functions. Something like:

infix notin = ∉

Or even better, the ability to create our own infix operators.

The way to do this with infix operators is either a \notin<tab> b or !(a in b). But if you’re willing to give up infix, you can use the function form !in(a,b).