b is a view similar to FIG. 15a showing more of the structure of which FIG. 15a is a part. [0027]FIG. 15c is a view similar to FIG. 15b showing the FIG. 15b structure in another operational condition. [0028]FIG. 15d is a simplified elevational view of an alternative structure which can be used to provide part of the apparatus shown in FIG. 15. [0029]FIG. 15e is a view similar to FIG. 15d showing t