reserve a,b,c,d for set,
  D,X1,X2,X3,X4 for non empty set,
  x1,y1,z1 for Element of X1,
  x2 for Element of X2,
  x3 for Element of X3,
  x4 for Element of X4,
  A1,B1 for Subset of X1;
reserve x,y for Element of [:X1,X2,X3:];
reserve x for Element of [:X1,X2,X3,X4:];
reserve A2 for Subset of X2,
  A3 for Subset of X3,
  A4 for Subset of X4;

theorem
  A1 = { x1 : x1 in A1 }
proof
  thus A1 c= { x1 : x1 in A1 };
  let a be object;
  assume a in { x1 : x1 in A1 };
  then ex x1 st a = x1 & x1 in A1;
  hence thesis;
end;
