theorem Th27:
  for D, E being set, F being FinSequence of D*, G being
FinSequence of E* st Card F = Card G holds len FlattenSeq F = len FlattenSeq G
proof
  let D, E be set, F be FinSequence of D*, G be FinSequence of E*;
  assume Card F = Card G;
  hence len FlattenSeq F = Sum Card G by Th26
    .= len FlattenSeq G by Th26;
end;
