The ornamental design for a beam, as shown and described.
FIG. 1 is a top right perspective view of the beam;
FIG. 2 is a top left perspective view of the beam;
FIG. 3 is a bottom left perspective view of the beam;
FIG. 4 is a top plan view of the beam;
FIG. 5 is a bottom plan view of the beam;
FIG. 6 is a right elevation view of the beam;
FIG. 7 is a left elevation view of the beam;
FIG. 8 is a front elevation view of the beam; and,
FIG. 9 is a rear elevation view of the beam.
Shade lines shown in the accompanying drawings of FIGS. 1-9 represent surface contour only and not texture, color shading, or surface ornamentation. The broken lines shown in FIGS. 1-7 illustrate portions of the beam that form no part of the claimed design. The beam is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.