let T be _Tree; :: thesis: for a, b, c being Vertex of T holds MiddleVertex (a,b,c) = MiddleVertex (c,a,b)

let a, b, c be Vertex of T; :: thesis: MiddleVertex (a,b,c) = MiddleVertex (c,a,b)

thus MiddleVertex (a,b,c) = MiddleVertex (a,c,b) by Th41

.= MiddleVertex (c,a,b) by Th42 ; :: thesis: verum

let a, b, c be Vertex of T; :: thesis: MiddleVertex (a,b,c) = MiddleVertex (c,a,b)

thus MiddleVertex (a,b,c) = MiddleVertex (a,c,b) by Th41

.= MiddleVertex (c,a,b) by Th42 ; :: thesis: verum