Convert an A1-style spreadsheet column label to a number.

ABC2NUM translates an A1-style (absolute) column label reference to a number; NUM2ABC does inverse.

ABC2NUM(S) returns the number corresponding to the spreadsheet column represented by the reference string, S. The input string, S, may contain the ascii letters A-Z, in the format commonly employed by spreadsheet programs such as Calc and Microsoft Excel[1]. Other characters, including lower case letters, a-z, are ignored.

NUM2ABC(D) returns the A1-style string label corresponding to the appropriate spreadsheet column represented by the specified integer, D. The specified integer must be greater than or equal to one.

references:
[1] Abraham, R., Burnett, M. and Erwig, M., "Spreadsheet Programming" Encyclopedia of Computer Science and Engineering, (ed. B.J. Wah), 2804-2810, 2009
DOI: 10.1002/9780470050118.ecse415

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