theorem Th69:
  congr a,b,c,d implies congr c,d,a,b & congr b,a,d,c & congr a,c,
  b,d & congr d,c,b,a & congr b,d,a,c & congr c,a,d,b & congr d,b,c,a
proof
  assume
A1: congr a,b,c,d;
  then congr c,d,a,b;
  then
A2: congr d,c,b,a by Th67;
  congr b,a,d,c & congr a,c,b,d by A1,Th67,Th68;
  hence thesis by A2,Th67,Th68;
end;
