@Kha I was tired of writing `arbitrary _` :)
There 0 places in the stdlib where the type needs to be provided.
If in the future we need to specify the type we can use
`arbitrary (α := <type>)`
Versions of `introN` and `intro1` that preserve the binder name.
They are used to implement the idiom: "revert", do something, re-"intro"-tuce
Before this commit `introNP mvarId n` was `intro1 mvarId n [] false`.