reserve a,b,i,k,m,n for Nat;
reserve s,z for non zero Nat;
reserve c for Complex;

theorem Th7:
  a = n-1 & k < n implies k = 0 or ... or k = a
  proof
    assume a = n-1 & k < n;
    then k <= a by INT_1:52;
    hence thesis;
  end;
