reserve x,y,z for set;
reserve f,f1,f2,f3 for FinSequence,
  p,p1,p2,p3 for set,
  i,k for Nat;
reserve D for non empty set,
  p,p1,p2,p3 for Element of D,
  f,f1,f2 for FinSequence of D;
reserve D for non empty set;
reserve p, q for FinSequence,
  X, Y, x, y for set,
  D for non empty set,
  i, j, k, l, m, n, r for Nat;

theorem Th14:
  1<=k & k<=len p implies (p^'q).k=p.k
proof
  assume that
A1: 1<=k and
A2: k<=len p;
  k in dom p by A1,A2,FINSEQ_3:25;
  hence thesis by FINSEQ_1:def 7;
end;
