The ornamental design for a set of coupling elements, as shown and described.
FIG. 1 is a right rear perspective view from above of a set of coupling elements embodying my new design, the left side being a mirror image of the right side;
FIG. 2 is a right rear perspective view from below thereof;
FIG. 3 is a left front perspective view from below thereof; and,
FIG. 4 is a left front perspective view from above of the set of coupling elements as assembled.