The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp in accordance with the present design;
FIG. 2 is a front view thereof, the rear view being a mirror image;
FIG. 3 is a left view thereof, the right view being a mirror image;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a top view thereof.
The broken lines shown in the drawings illustrate portions of the lamp that form no part of the claimed design. Any shading forms no part of the claimed design and is provided to illustrate contour only and is not an indication of texture, gloss, and the like.