for G being Graph
for c being Chain of G st c alternates_vertices_in G holds
ex vs1, vs2 being FinSequence of the carrier of G st
( vs1 <> vs2 & vs1 is_vertex_seq_of c & vs2 is_vertex_seq_of c & ( for vs being FinSequence of the carrier of G holds
( not vs is_vertex_seq_of c or vs = vs1 or vs = vs2 ) ) )