When elastomeric yarns are unwound and fed to the needles of a circular knitting apparatus, a yarn package relaxation profile is set, or determined, with corresponding adjustments made to the unwinding speed based on such package relaxation profile. A glass substance which has a sag point not higher than 565 C. The cyclic phosphonic anhydride preferably used is a 2,4,6-substituted 1,3,5,2,4,6-trioxatriphosphinane-2,4,6-trioxide of formula, wherein R\u2032 independently represents allyl, aryl or open-chain or branched C1 to C12 alkyl groups. Rotation of the hydraulic steering motor is selectively managed by a steering control valve attached to a cab-mounted windrower steering wheel or a proportional steering valve managed by the autoguidance system. The delayed clock signal and the other clock signal are provided as the first and second output clock signals. Thus, each of the spreaders and the ribs can be removed from the movable ring and the fixed ring respectively for replacement or maintenance, thereby enhancing the lifetime of the umbrella.