Please explain this example in the Stream docs, what is [< >] syntax?

Question

https://ocaml.org/api/Stream.html

val from : (int -> 'a option) -> 'a t

Stream.from f returns a stream built from the function f. To create a new stream element, the function f is called with the current stream count. The user function f must return either Some <value> for a value or None to specify the end of the stream.

Do note that the indices passed to f may not start at 0 in the general case. For example, [< '0; '1; Stream.from f >] would call f the first time with count 2.

There are two things confusing me about this example.

1.

I had no luck googling for meaning of [< ... >] syntax. The closest I found was: https://ocaml.org/manual/lex.html#sss:keywords which just says those character sequences are keywords

[< ... ] seems to be used when printing, but not defining, polymorphic variants: https://ocaml.org/manual/polyvariant.html

If I paste in something like [< '0; '1; >] I get a syntax error.

So it's currently quite baffling to me what this example is purporting to show.

2.

The example says that [< '0; '1; Stream.from f >] would call f the first time with count 2

And I just wonder ... why? how? I can see that 2 follows on from '0 and '1, but how do those values influence starting value of f? (and why are they prefixed with '?)