:: deftheorem defines being_special_polygon TOPREAL4:def 2 :
for P being Subset of (TOP-REAL 2) holds
( P is being_special_polygon iff ex p1, p2 being Point of (TOP-REAL 2) ex P1, P2 being Subset of (TOP-REAL 2) st
( p1 <> p2 & p1 in P & p2 in P & P1 is_S-P_arc_joining p1,p2 & P2 is_S-P_arc_joining p1,p2 & P1 /\ P2 = {p1,p2} & P = P1 \/ P2 ) );