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* Algorithm AS 226 Appl. Statist. (1987) Vol. 36, No. 2
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* Incorporates modification AS R84 from AS Vol. 39, pp311-2, 1990
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* and AS R95 from AS Vol. 44, pp551-2, 1995
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* original (C) Royal Statistical Society 1987, 1990, 1995
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* Returns the cumulative probability of x for the non-central