module Interval_intf:sig..end
module type Gen =sig..end
module type Gen_set =sig..end
module type S =sig..end
module type S1 =sig..end
create l u returns the interval with lower bound l and upper bound u, unless
      l > u, in which case create returns the empty interval.convex_hull ts returns an interval whose upperbound is the greatest upperbound of
      the intervals in the list, and whose lowerbound is the least lowerbound of the
      list.bound t x returns None iff is_empty t.  If bounds t = Some (a, b), then
      bound returns Some y where y is the element of t closest to x.  I.e.:
      |  y = a  if x < a
      |  y = x  if a <= x <= b
      |  y = b  if x > b
is_superset i1 of_:i2 is whether i1 contains i2.  The empty interval is
      contained in every interval.
map t ~f returns create (f l) (f u) if bounds t = Some (l, u), and empty if
      t is empty.  Note that if f l > f u, the result of map is empty, by the
      definition of create.
      If one thinks of an interval as a set of points, rather than a pair of its bounds,
      then map is not the same as the usual mathematical notion of mapping f over that
      set.  For example, ~f:(fun x -> x * x) maps the interval 
[-1,1]to
[1,1]
     , not to [0,1].
are_disjoint ts returns true iff the intervals in ts are pairwise disjoint.ilist1 and ilist2 are lists of (disjoint) intervals,
      list_intersect ilist1 ilist2 returns the list of disjoint intervals that
      correspond to the intersection of ilist1 with ilist2.