Something like Roman numerals for math geeks. Popular with Lisp lovers and one more reason for WhyWeHateLisp.

	* http://web.mat.bham.ac.uk/R.W.Kaye/pa.htm -- "difficult reading" quote
	* http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=peano

It is a formalisation of the well-known natural numbers (non-negative integers). Natural numbers are very handy for counting. Peano showed that we don't have to be afraid to run out of them, since there are infinitely many.
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Is its popularity with Lisp lovers primarily due to its use in StructureAndInterpretationOfComputerPrograms?
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CategoryMath