IEEE hexadecimal to double precision number conversion.

  HEX2NUM(S), where S is a 16 character string containing

  a hexadecimal number, returns the IEEE double precision

  floating point number it represents.  Fewer than 16

  characters are padded on the right with zeros.



  For example, HEX2NUM('400921fb54442d18') returns Pi.

               HEX2NUM('bff') returns -1.



  NaNs, infinities and denorms are handled correctly.  See

  also FORMAT478570 HEX.