We claim the ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view showing another embodiment of our design for a lamp.
FIG. 2 is a side view of the embodiment of our design shown in FIG. 1. Again, all sides of the lamp appear the same, save for broken line portions, which are directed to environmental features. Therefore this is the only side view shown.
FIG. 3 is a top view of the embodiment of our design shown in FIG. 1; and,
FIG. 4 is a bottom view of the embodiment of our design as shown in FIG. 1.