Early inference engines focused primarily on forward chaining. These systems were usually implemented in the Lisp programming language. Lisp was a frequent platform for early AI research due to its strong capability to do symbolic manipulation. Also, as an interpreted language it offered productive development environments appropriate to debugging complex programs. A necessary consequence of these benefits was that Lisp programs tended to be slower and less robust than compiled languages of the time such as C. A common approach in these early days was to take an expert system application and repackage the inference engine used for that system as a re-usable tool other researchers could use for the development of other expert systems. For example, MYCIN was an early expert system for medical diagnosis and EMYCIN was an inference engine extrapolated from MYCIN and made available for other researchers.