Greenspun's tenth rule
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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 features required to develop such complexity in other programming languages are equivalent to the methods used by Lisp.
It can also be interpreted as a satirical critique of systems that include complex, highly configurable sub-systems. Rather than including a custom interpreter for some domain-specific language, Greenspun's rule suggests using a widely accepted, fully featured language like Lisp.
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 correlary jokingly suggests that even Lisp is slow, buggy, and/or incomplete.