DATATYPE(string[,option])

This function tests the datatype of the string. If the function is called
with just one argument, then the function returns "NUM" if the argument
is a valid number, and "CHAR" otherwise.

If both arguments are supplied, then the answer is a logical value
(0 or 1) indicating whether or not string is of the type given by the
option.  Is s is an empty string then the answer is always 0 unless
testing for a hex constant (the null string is a valid hex constant).
The option must be one of the following:

   'A': s is alphanumeric (containing characters only from the ranges
        "a-z", "A-Z" and "0-9")
   'B': s contains bits (all characters 0 or 1)
   'L': s is lower case (containing only "a-z")
   'M': s is mixed case (containing only "a-z" and "A-Z")
   'N': s is a valid number
   'S': s contains characters valid in symbols (variables)
   'U': s is upper case (containing only "A-Z")
   'W': s is a whole number
   'X': s satisfies the rules for a hex constant (i.e. contains only
        "0-9", "a-f", "A-F" and blanks in appropriate places).