It's difficult to search for but wondering what `<>` means as in [here](https://stackoverflow.com/questions/54937970/how-to-define-axiom-of-a-line-as-two-points-in-coq/54938069#54938069):

```
Axiom point : Type.
Axiom line  : Type.
Axiom lies_in : point -> line -> Prop.
Axiom ax : forall (p1 p2 : point), p1 <> p2 ->
           exists! l : line, lies_in p1 l /\ lies_in p2 l.

```