theorem Th51: :: HELLY:51

for T being _Tree
for P1, P2 being Path of T
for a, b, c being Vertex of T st P1 = T .pathBetween (a,b) & P2 = T .pathBetween (a,c) & not b in P2 .vertices() & not c in P1 .vertices() holds
MiddleVertex (a,b,c) = P1 . (len (maxPrefix (P1,P2)))