
theorem Th4:
  for G being _Graph, T being Trail of G holds T.length() = card(T.edges())
proof
  let G be _Graph, T be Trail of G;
  A1: T.edgeSeq() is one-to-one by GLIB_001:def 27;
  thus T.length() = card dom(T.edgeSeq()) by CARD_1:62
    .= card(T.edges()) by A1, CARD_1:70;
end;
