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

theorem Th1:
  i>=len p implies p.i=0.R
proof
  assume
A1: i>=len p;
  len p is_at_least_length_of p by Def3;
  hence thesis by A1;
end;
