reserve E,F,G for RealNormSpace;
reserve f for Function of E,F;
reserve g for Function of F,G;
reserve a,b,c for Point of E;
reserve t for Real;

theorem Th4:
  a + b - b = a
  proof
    thus a + b - b = a + (b - b) by RLVECT_1:28
    .= a + 0.E by RLVECT_1:15
    .= a;
  end;
