reserve x,y for object,
        D,D1,D2 for non empty set,
        i,j,k,m,n for Nat,
        f,g for FinSequence of D*,
        f1 for FinSequence of D1*,
        f2 for FinSequence of D2*;
reserve f for complex-valued Function,
        g,h for complex-valued FinSequence;

theorem Th22:
  Sum h = h.1 + (h,2) +...
proof
  Sum h = (h,1)+... by Th18
       .= h.1+(h,1+1)+... by Th20;
  hence thesis;
end;
