The ornamental design for a lamp, as shown and described.
FIG. 1 is a perspective view of a lamp showing the claimed new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a detail view of area 6 in FIG. 5.
The broken lines immediately adjacent to the claimed portions are boundaries, all other broken lines are environmental structures; all the broken lines form no part of the claimed design.