reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;

theorem Th48:
  {x,y} misses Z implies not x in Z
proof
A1: x in {x,y} by TARSKI:def 2;
  assume {x,y} /\ Z = {} & x in Z;
  hence contradiction by A1,XBOOLE_0:def 4;
end;
