I claim the ornamental design for a lamp, as shown and described.
FIG. 1 is a front elevation view of a lamp showing my new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 Is a top plan view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top and left side perspective view thereof.
The broken-line disclosure shown in FIGS. 1 and 6 is understood to represent a portion of the article in which the claimed design is embodied, but which forms no part of the claimed design.