theorem
  Leaves root-tree c = {c}
  proof
    thus Leaves root-tree c = (root-tree c).:Leaves elementary_tree 0
    .= ({{}}-->c).:dom({{}}-->c) by TREES_1:29,HUFFMAN1:15
    .= rng ({{}}-->c) by RELAT_1:113 .= {c} by FUNCOP_1:8;
  end;
