reserve v,x,x1,x2,x3,x4,y,y1,y2,y3,y4,z,z1,z2 for object,
  X,X1,X2,X3,X4,Y,Y1,Y2,Y3,Y4,Y5,
  Z,Z1,Z2,Z3,Z4,Z5 for set;
reserve p for pair object;
reserve R for Relation;
reserve xx1 for Element of X1,
  xx2 for Element of X2,
  xx3 for Element of X3;
reserve xx4 for Element of X4;
reserve A1 for Subset of X1,
  A2 for Subset of X2,
  A3 for Subset of X3,
  A4 for Subset of X4;
reserve x for Element of [:X1,X2,X3:];
reserve x for Element of [:X1,X2,X3,X4:];
reserve x for object;

theorem
  [[x1,x2],y]`11 = x1 & [[x1,x2],y]`12 = x2 & [x,[y1,y2]]`21 = y1 & [x,[
  y1,y2]]`22 = y2;
