reserve i,j,k,n for Nat;
reserve D for non empty set,
  p for Element of D,
  f,g for FinSequence of D;

theorem Th19:
  for f being FinSequence, i being Nat holds rng(f|i) c= rng f
proof
  let f be FinSequence, i be Nat;
  f|i c= f by RELAT_1:59;
  hence thesis by RELAT_1:11;
end;
