The ornamental design for a part of an armchair, as shown and described.
FIG. 1 is a perspective view of an armchair showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof, opposite that of FIG. 3;
FIG. 5 is a side elevational view thereof;
FIG. 6 is a side elevational view thereof, from a side opposite that of FIG. 5;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines showing of part of the armchair in the figures are included for the purpose of illustrating environmental structure and forms no part of the claimed design. The dash-dot-dash lines showing in the figures are included to illustrate the boundary of the claimed design.