reserve X,Y,Z,X1,X2,X3,X4,X5,X6 for set, x,y for object;
reserve a,b,c for object, X,Y,Z,x,y,z for set;
reserve A,B,C,D for Ordinal;

theorem Th6:
  for A,B being set, C being epsilon-transitive set st A in B & B in C
   holds A in C
proof
  let A,B be set, C be epsilon-transitive set;
  assume that
A1: A in B and
A2: B in C;
  B c= C by A2,Def2;
  hence thesis by A1;
end;
