reserve m,n for non zero Element of NAT;
reserve i,j,k for Element of NAT;
reserve Z for set;

theorem Th3:
  for f be PartFunc of REAL i,REAL holds dom <>*f = dom f
proof
   let f be PartFunc of REAL i,REAL;
   rng f c= dom (proj(1,1) qua Function") by PDIFF_1:2;
   hence thesis by RELAT_1:27;
end;
