code: public Quaternion fromAnglesXY (float x, float y) {
          float hx = x * 0.5f, hy = y * 0.5f;
          float sx = FloatMath.sin(hx), cx = FloatMath.cos(hx);
          float sy = FloatMath.sin(hy), cy = FloatMath.cos(hy);
          return set(cy*sx, sy*cx, -sy*sx, cy*cx);
      }
docs: Sets this quaternion to one that first rotates about x by the specified number of radians,
  then rotates about y by the specified number of radians.