A new function ++INCLUDE'ONCE <file> acts same as ++INCLUDE except that if a file with the same name and extension (not considering ppn, ersatz, disk, etc.) has already been included, the operation is ignored. This turns out to be quite convenient if you store user-defined functions in individual BSI files, and some of them reference other user-defined functions. This way, you can ++INCLUDE'ONCE the BSI's needed by the current BSI, without having to know whether the calling program has already ++INCLUDEd that BSI. (Of course, this only works if the main program also uses ++INCLUDE'ONCE for this type of BSI.)
