+.Ss \&OP
+Optional command-line argument.
+This is a non-standard GNU extension, included only for compatibility.
+It has the following syntax:
+.Bd -filled -offset indent
+.Pf \. Sx \&OP
+.Cm key Op Cm value
+.Ed
+.Pp
+The
+.Cm key
+is usually a command-line flag and
+.Cm value
+its argument.