The ornamental design for an anchor, as shown and described.
FIG. 1 is a bottom, left perspective view of an anchor showing our new design;
FIG. 2 is a top, left perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a front elevational view thereof.
The broken line showing of structural elements is included for the purpose of illustrating portions of the anchor that form no part of the claimed design.