First of all, any two vectors OC, OP, if vector OC+ vector OP= vector OD, and OC and OP are not * * * straight lines, then the quadrilateral OPDC must be a parallelogram. The proof is simple. The moving vector OP makes O and C coincide. Remember that P moves to point Q, and OP is parallel to CQ, so the quadrilateral OPQC is a parallelogram, and vector OC+ vector OP= vector OC+ vector CQ= vector OQ. Then OP=OC, the parallelogram and adjacent sides are equal, so the quadrilateral OPDC must be a diamond.