Support for multiple occurrences of the same data set name

The existence of multiple occurrences of the same data set name, involving different sets of volumes, is supported. The most recently created occurrence of a data set name is considered to be the default (or cataloged) version of the data set; i.e., if a non-scratch tape mount command is issued for a data set name and a VOLSER is not specified, a volume lookup will occur for the most recently created occurrence of the data set.