let C be Simple_closed_curve; :: thesis: for p1, p2, q being Point of () st q in C & p1 in C & p2 in C & p1 <> p2 & p1 <> q & p2 <> q holds
p1,p2 are_neighbours_wrt q,q,C

let p1, p2, q be Point of (); :: thesis: ( q in C & p1 in C & p2 in C & p1 <> p2 & p1 <> q & p2 <> q implies p1,p2 are_neighbours_wrt q,q,C )
assume that
A1: q in C and
A2: ( p1 in C & p2 in C & p1 <> p2 ) and
A3: ( p1 <> q & p2 <> q ) ; :: thesis: p1,p2 are_neighbours_wrt q,q,C
consider P1, P2 being non empty Subset of () such that
A4: P1 is_an_arc_of p1,p2 and
A5: P2 is_an_arc_of p1,p2 and
A6: C = P1 \/ P2 and
A7: P1 /\ P2 = {p1,p2} by ;
per cases ( ( q in P1 & not q in P2 ) or ( q in P2 & not q in P1 ) or ( q in P1 & q in P2 ) ) by ;
suppose A8: ( q in P1 & not q in P2 ) ; :: thesis: p1,p2 are_neighbours_wrt q,q,C
take P2 ; :: according to JORDAN18:def 3 :: thesis: ( P2 is_an_arc_of p1,p2 & P2 c= C & not P2 meets {q,q} )
thus P2 is_an_arc_of p1,p2 by A5; :: thesis: ( P2 c= C & not P2 meets {q,q} )
thus P2 c= C by ; :: thesis: not P2 meets {q,q}
thus P2 misses {q,q} by ; :: thesis: verum
end;
suppose A9: ( q in P2 & not q in P1 ) ; :: thesis: p1,p2 are_neighbours_wrt q,q,C
take P1 ; :: according to JORDAN18:def 3 :: thesis: ( P1 is_an_arc_of p1,p2 & P1 c= C & not P1 meets {q,q} )
thus P1 is_an_arc_of p1,p2 by A4; :: thesis: ( P1 c= C & not P1 meets {q,q} )
thus P1 c= C by ; :: thesis: not P1 meets {q,q}
thus P1 misses {q,q} by ; :: thesis: verum
end;
suppose ( q in P1 & q in P2 ) ; :: thesis: p1,p2 are_neighbours_wrt q,q,C
end;
end;