reserve i,j,n,n1,n2,m,k,l,u for Nat,
        r,r1,r2 for Real,
        x,y for Integer,
        a,b for non trivial Nat,
        F for XFinSequence,
        cF,cF1,cF2 for complex-valued XFinSequence,
        c,c1,c2 for Complex;

theorem Th4:
  cF = {} implies Product cF = 1
  proof
    assume cF={};
    then len cF = 0;
    hence thesis by BINOP_2:6,AFINSQ_2:def 8;
  end;
