reserve i,k,l,m,n for Nat,
  x for set;
reserve R for non empty ZeroStr;
reserve p,q for AlgSequence of R;
reserve x for Element of R;

theorem Th7:
  <%0.R%>.i=0.R
proof
  set p0=<%0.R%>;
  now
    assume i<>0;
    then i>0 by NAT_1:3;
    then i>=len p0 by Th6;
    hence thesis by Th1;
  end;
  hence thesis by Def4;
end;
