QBLDSYS

The QBLDSYS is "an IBM-internal library that provides the capability to build the OS and LPPs; there should be an expectation, barring any radically daft changes, that a library by that name would never exist on a customer system".

There is also a (presumably related) library called QBLDSYSR, various references to which may be found through a web search; how exactly QBLDSYSR relates to QBLDSYS is unclear.