DEC2BASE27(D) returns the representation of D as a string in base 27,
expressed as 'A'..'Z', 'AA','AB'...'AZ', until 'IV'. Note, there is no zero
digit, so strictly we have hybrid base26, base27 number system. D must be a
negative integer bigger than 0 and smaller than 2^52, which is the maximum
number of columns in an Excel worksheet.
Examples
dec2base(1) returns 'A'
dec2base(26) returns 'Z'
dec2base(27) returns 'AA'
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