@startdot digraph G { node [fontsize = 12, shape = box, style=rounded ]; a [label="pならばqである"] b [label="qならばpである"] c [label="pでないならばqでない"] d [label="qでないならばpでない"] a -> b [label = "逆" dir = "both"]; a -> c [label = "裏" dir = "both"]; a -> d [label = "対偶" dir = "both"]; c -> d [label = "逆" dir = "both"]; b -> d [label = "裏" dir = "both"]; b -> c [dir = "both"]; {rank = same; a; b; } {rank = same; c; d; } } @enddot