The function fails if one of the arguments is NaN. This causes an
    exception. To avoid this, C99 defines the macro islessequal (x, y),
    which determines (x) <= (y) without an exception if x or y is NaN.

    The macro is guaranteed to evaluate the arguments only once. The
    arguments must be of real floating-point type.

    Note:
    Do not pass integer values as arguments to this macro because the
    arguments will not be promoted to real floating-point types.