reserve a,b,c,d,m,x,n,j,k,l for Nat,
  t,u,v,z for Integer,
  f,F for FinSequence of NAT;
reserve p,q,r,s for real number;

theorem Th88:
  (t + (u mod z))mod z = (t+u) mod z
  proof
    thus (t+(u mod z)) mod z = ((t mod z + ((u mod z) mod z)) mod z) mod z
      by NAT_D:66
    .= (t+u) mod z by NAT_D:66;
  end;
