reserve W,X,Y,Z for set,
  f,g for Function,
  a,x,y,z for set;
reserve u,v for Element of Tarski-Class(X),
  A,B,C for Ordinal,
  L for Sequence;

theorem
  x in Rank A & y in Rank A iff [x,y] in Rank succ succ A
proof
A1: x in Rank A iff {x} in Rank succ A by Th43;
 x in Rank A & y in Rank A iff {x,y} in Rank succ A by Th44;
  hence thesis by A1,Th44;
end;
