theorem Th107:
  for f being FinSequence st len f = m+1 & n in dom f holds len Del(f,n) = m
proof
  let f be FinSequence such that
A1: len f = m+1 and
A2: n in dom f;
  ex k being Nat st len f = k+1 & len Del(f,n) = k by A2,Th102;
  hence thesis by A1;
end;
