I'd like to define a class in Ocaml without giving intial values to its fields. Is it possible ?

For example, as far as I've seen, a class with a int and an int list fields would look like this:

class dummy () =
	val mutable v = 0
	val mutable l = ([] : int list)
end;;


but what about a field of type t, for which there is no default value ?
i.e.

	val mutable v = ( ??? : t )


other questions:
- are documentations available in pdf (Acrobat Reader) format ?
- there is no 'per line' comment (like Ada's "--" or C++'s "//" ) ?