The ornamental design for a lamp, as shown and described.
FIG. 1 is a front-left top perspective view of a lamp according to the present invention;
FIG. 2 is a rear-left top perspective view of the lamp thereof;
FIG. 3 is a front plan view of the lamp thereof;
FIG. 4 is a rear plan view of the lamp thereof;
FIG. 5 is a right-side plan view of the lamp thereof;
FIG. 6 is a left-side plan view of the lamp thereof;
FIG. 7 is a top plan view of the lamp thereof; and,
FIG. 8 is a bottom plan view of the lamp thereof.
The dashed lines formed by segments of equal length represent portions of the lamp which form no part of the claimed design, and the dash-dot-dash lines represent an unclaimed boundary between claimed and unclaimed surfaces.