Return a string describing the class of the operand.
The returned value is one of the following ten strings:
* '-Infinity', indicating that the operand is negative infinity.
* '-Normal', indicating that the operand is a negative normal
number.
* '-Subnormal', indicating that the operand is negative and
subnormal.
* '-Zero', indicating that the operand is a negative zero.
* '+Zero', indicating that the operand is a positive zero.
* '+Subnormal', indicating that the operand is positive and
subnormal.
* '+Normal', indicating that the operand is a positive normal
number.
* '+Infinity', indicating that the operand is positive infinity.
* 'NaN', indicating that the operand is a quiet NaN (Not a
Number).
* 'sNaN', indicating that the operand is a signaling NaN.