Synopsis:

       answer = fp_sub(number1, number2[, decimals])

Arguments:

       number1 - The first floating point number

       number2 - The second floating point number

       [decimals] - Optional argument to specify the number of decimal places of the returned result

Description:   This function subtracts number2 from number1 and returns the result.    If the optional decimals argument is specified then the result will be returned with this number of decimal places,  otherwise the global default number of decimal places will be used (as specified by fp_decs()).

Returns:   Returns the result of subtracting  number2 from number1