Aspects of the language were still being designed as PL/I F was implemented, so some were omitted until later releases. PL/I RECORD I/O was shipped with PL/I F Release 2. The list processing functions Based Variables, Pointers, Areas and Offsets and LOCATE-mode I/O were first shipped in Release 4. In a major attempt to speed up PL/I code to compete with Fortran object code, PL/I F Release 5 does substantial program optimization of DO-loops facilitated by the REORDER option on procedures.