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;

theorem Th15:
  x in R implies x = [x`1,x`2]
proof
  assume x in R;
  then ex x1,x2 being object st x=[x1,x2] by RELAT_1:def 1;
  hence thesis;
end;
