add6 method
Implementation
void add6(double a, double b, double c, double d, double e, double f) {
_ensure(6);
final p = _data, n = length;
p[n] = a;
p[n + 1] = b;
p[n + 2] = c;
p[n + 3] = d;
p[n + 4] = e;
p[n + 5] = f;
length += 6;
}