In parallel with that Steve Hardy at Sussex University implemented a subset of POP-2, which he called POP-11 which ran on a DEC PDP-11/40 computer. It was originally designed to run on the DEC operating system RSX-11D, in time-shared mode for teaching, but that caused so many problems that an early version of Unix was installed and used instead. That version of Pop-11 was written in Unix assembler, and code was incrementally compiled to an intermediate byte code which was interpreted. That port was completed around 1976 and as a result Pop-11 was used in several places for teaching. In order to support its teaching function many of the syntactic features of POP-2 were modified, e.g. replacing function ... end with define ... enddefine and adding a wider variety of looping constructs with closing brackets to match their opening brackets instead of the use of close for all loops in POP-2. Pop-11 also introduced a pattern matcher for list structures, making it much easier to teach AI programming.