cell containment discrepancy about geo HOT 4 CLOSED

  // Construct a loop corresponding to the given cell.
  //
  // Note that the loop and cell *do not* contain exactly the same set of
  // points, because S2Loop and S2Cell have slightly different definitions of
  // point containment.  For example, an S2Cell vertex is contained by all
  // four neighboring S2Cells, but it is contained by exactly one of four
  // S2Loops constructed from those cells.  As another example, the S2Cell
  // coverings of "cell" and "S2Loop(cell)" will be different, because the
  // loop contains points on its boundary that actually belong to other cells
  // (i.e., the covering will include a layer of neighboring cells).
  explicit S2Loop(const S2Cell& cell);

// Point containment of loops is defined such that if the sphere is subdivided
// into faces (loops), every point is contained by exactly one face.  This
// implies that loops do not necessarily contain their vertices.
//
// Note: The reason that duplicate vertices and intersecting edges are not
// allowed is that they make it harder to define and implement loop
// relationships, e.g. whether one loop contains another.  If your data does
// not satisfy these restrictions, you can use S2Builder to normalize it.