The PL/I Optimizer and Checkout compilers produced in Hursley support a common level of PL/I language and aimed to replace the PL/I F compiler. The checkout compiler is a rewrite of PL/I F in BSL, IBM's PL/I-like proprietary implementation language (later PL/S). The performance objectives set for the compilers are shown in an IBM presentation to the BCS. The compilers had to produce identical results the Checkout Compiler is used to debug programs that would then be submitted to the Optimizer. Given that the compilers had entirely different designs and were handling the full PL/I language this goal was challenging: it was achieved.