selfAdd method
Adds the argument to the value of this. To prevent altering constants, this method must only be used on values known to be newly created.
@param y the addend @return this object, increased by y
Implementation
DD selfAdd(double y) {
double H, h, S, s, e, f;
S = hi + y;
e = S - hi;
s = S - e;
s = (y - e) + (hi - s);
f = s + lo;
H = S + f;
h = f + (S - H);
hi = H + h;
lo = h + (H - hi);
return this;
// return selfAdd(y, 0.0);
}