ifier">k</span></code>
                  is in the map already, the intersection function is propagated
                  to the associated value, if it exists for the Map's codomain_type.
                </p>
                <p>
                  If the codomain_type has no intersection operation, associated
                  values are combined using addition. For partial map types this
                  results in an addition on the intersection of the domains of the
                  intersected sets. F