/**
 * Determines if a time interval is empty.
 *
 * @name czm_isEmpty
 * @glslFunction 
 * 
 * @param {czm_raySegment} interval The interval to test.
 * 
 * @returns {bool} <code>true</code> if the time interval is empty; otherwise, <code>false</code>.
 *
 * @example
 * bool b0 = czm_isEmpty(czm_emptyRaySegment);      // true
 * bool b1 = czm_isEmpty(czm_raySegment(0.0, 1.0)); // false
 * bool b2 = czm_isEmpty(czm_raySegment(1.0, 1.0)); // false, contains 1.0.
 */
bool czm_isEmpty(czm_raySegment interval)
{
    return (interval.stop < 0.0);
}