theorem Th53: :: HELLY:53

for T being _Tree
for a, b, c being Vertex of T
for S being non empty set st ( for s being set st s in S holds
( ex t being _Subtree of T st s = the_Vertices_of t & ( ( a in s & b in s ) or ( a in s & c in s ) or ( b in s & c in s ) ) ) ) holds
meet S <> {}