
theorem :: PMV104: :: PMV(a,b,c) = PMV(c,b,a)
  for T being _Tree for a, b, c being Vertex of T holds MiddleVertex(a,b
  ,c) = MiddleVertex(c,b,a)
proof
  let T be _Tree;
  let a, b, c be Vertex of T;
  thus MiddleVertex(a,b,c) = MiddleVertex(c,a,b) by Th44
    .= MiddleVertex(c,b,a) by Th41;
end;
