Greenspun's tenth rule
This expresses the opinion that the perceived flexibility and extensibility designed into the Lisp programming language includes all functionality that is theoretically necessary to write a complex computer program, and that the core implementations of other programming languages often do not supply critical functionality necessary to develop complex programs.
The rule was written sometime around 1993 by Philip Greenspun. Although it is known as his tenth rule, there are in fact no preceding rules, only the tenth. The reason for this according to Greenspun:
…including Common Lisp.
This can be viewed as a commentary on the difficulty of creating an efficient implementation of the large and complex Common Lisp language, or simply a joke about Lisp's eval function. Both Greenspun's rule and Morris's corollary are examples of a characteristic style of hacker humor known as "ha ha only serious".