let P be Subset of (); :: thesis: for p1, p2, q being Point of () st P is_an_arc_of p1,p2 holds
L_Segment (P,p1,p2,q) = R_Segment (P,p2,p1,q)

let p1, p2, q be Point of (); :: thesis: ( P is_an_arc_of p1,p2 implies L_Segment (P,p1,p2,q) = R_Segment (P,p2,p1,q) )
assume A1: P is_an_arc_of p1,p2 ; :: thesis: L_Segment (P,p1,p2,q) = R_Segment (P,p2,p1,q)
thus L_Segment (P,p1,p2,q) c= R_Segment (P,p2,p1,q) :: according to XBOOLE_0:def 10 :: thesis: R_Segment (P,p2,p1,q) c= L_Segment (P,p1,p2,q)
proof
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in L_Segment (P,p1,p2,q) or x in R_Segment (P,p2,p1,q) )
assume x in L_Segment (P,p1,p2,q) ; :: thesis: x in R_Segment (P,p2,p1,q)
then consider p being Point of () such that
A2: p = x and
A3: LE p,q,P,p1,p2 ;
LE q,p,P,p2,p1 by A1, A3, Th18;
hence x in R_Segment (P,p2,p1,q) by A2; :: thesis: verum
end;
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in R_Segment (P,p2,p1,q) or x in L_Segment (P,p1,p2,q) )
assume x in R_Segment (P,p2,p1,q) ; :: thesis: x in L_Segment (P,p1,p2,q)
then consider p being Point of () such that
A4: p = x and
A5: LE q,p,P,p2,p1 ;
LE p,q,P,p1,p2 by ;
hence x in L_Segment (P,p1,p2,q) by A4; :: thesis: verum