reserve a,b,p,x,x9,x1,x19,x2,y,y9,y1,y19,y2,z,z9,z1,z2 for object,
   X,X9,Y,Y9,Z,Z9 for set;
reserve A,D,D9 for non empty set;
reserve f,g,h for Function;

theorem Th12:
 for x being object holds
  x in dom(f +* g) iff x in dom f or x in dom g
proof let x be object;
  x in dom(f +* g) iff x in dom f \/ dom g by Def1;
  hence thesis by XBOOLE_0:def 3;
end;
