Deleted the redundant theorem subset_empty_iff (subset_empty exists already)
Deleted the redundant theorem subset_empty_iff (subset_empty exists already)

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle