NaN 
and INF. If a Prolog float is converted into a string it 
returns the XML canonical form. This form always has one digit before 
the decimal dot, at least one digit after it and an exponential 
component using the capital
E. This predicate behaves as number_string/2 
for integers.
Throws a syntax_error(xsd_number) if String 
is given and is not a well-formed XSD number.