Coupler

FIG. 1 is a top front left side perspective view of a coupler, showing the new design; FIG. 2 is a top plan view thereof; FIG. 3 is a bottom plan view thereof; FIG. 4 is a right side view thereof; FIG. 5 is a left side view thereof; FIG. 6 is a front view thereof; and, FIG. 7 is a rear view thereof. Thin evenly broken lines in all the figures depict portions of the coupler which form no part of the claimed design.