
theorem Th28:
  for T being _Tree, a, b being Vertex of T holds T.pathBetween(a,
  b).first() = a & T.pathBetween(a,b).last() = b
proof
  let T be _Tree, a, b be Vertex of T;
A1: T.pathBetween(a,b) is_Walk_from a, b by Def2;
  hence T.pathBetween(a,b).first() = a;
  thus thesis by A1;
end;
