create
creates an interval set containing intervals whose lower and upper bounds
are given by the pairs passed to the function. It is an error if the pairs overlap.
create_from_intervals
creates an interval set. Empty intervals are dropped. It is
an error if the nonempty intervals are not disjoint.
contains_set
returns true iff for every interval in the contained set, there
exists an interval in the container set that is its superset.