reserve x,y,z,c for object,
  f for Function of 2-tuples_on BOOLEAN, BOOLEAN;

theorem Th60:
  x in the carrier of 2GatesCircStr(x,y,c,f) & y in the carrier of
  2GatesCircStr(x,y,c,f) & c in the carrier of 2GatesCircStr(x,y,c,f)
proof
  set S1 = 1GateCircStr(<*x,y*>, f);
  set S2 = 1GateCircStr(<*[<*x,y*>, f], c*>, f);
A1: c in the carrier of S2 by Th43;
  x in the carrier of S1 & y in the carrier of S1 by Th43;
  hence thesis by A1,Th20;
end;
