class type propagator =object..end
method propagate : string -> string -> int64propagate name value version: push the new value of the variable
called name to other processes. The version of the new value is
returned.
method update : string -> int64 -> (string * int64) optionupdate name version: checks whether there is a new value of
the variable name with a version higher than the passed
version. If not, None is returned. If so, Some(val,vers)
is returned where val is the value with version vers.
