Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.
Overview
The rule expresses the opinion that the argued flexibility and extensibility designed into the programming language Lisp includes all functionality that is theoretically needed to write any complex computer program, and that the features required to develop and manage such complexity in other programming languages are equivalent to some subset of the methods used in Lisp.
Other programming languages, while claiming to be simpler, require programmers to reinvent in a haphazard way a significant amount of needed functionality that is present in Lisp as a standard, time-proven base.
Greenspun's tenth rule
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