theorem
  for D being set, d be Element of D^omega holds FlattenSeq <%d%> = d
proof
  let D be set, d be Element of D^omega;
  ex g being BinOp of D^omega st
  (for p, q being Element of D^omega holds g.(p,q) = p^q) &
  FlattenSeq <%d%> = g "**" <% d %> by Def10;
  hence thesis by Th37;
end;
