reserve G for non empty DTConstrStr,
  s for Symbol of G,
  n,m for String of G;
reserve n1,n2,n3 for String of G;

theorem Th6:
  n is_derivable_from n
proof
  take p = <*n*>;
  len p = 1 by FINSEQ_1:40;
  hence len p >= 1 & p.1 = n & p.len p = n;
  let i be Element of NAT;
  assume that
A1: i >= 1 and
A2: i < len p;
  thus thesis by A1,A2,FINSEQ_1:40;
end;
