reserve M for non empty set;
reserve V for ComplexNormSpace;
reserve f,f1,f2,f3 for PartFunc of M,V;
reserve z,z1,z2 for Complex;

theorem
  f1 - (-f2) = f1 + f2
proof
  thus f1 - (-f2) = f1 + (-(-f2)) by Th25
    .= f1 + f2 by Th24;
end;
