let V be non trivial RealLinearSpace; :: thesis: for p, p1, p2, r, r1 being Element of () st p,p1,p2 are_collinear & p,p1,r are_collinear & p1,p2,r1 are_collinear holds
ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

let p, p1, p2, r, r1 be Element of (); :: thesis: ( p,p1,p2 are_collinear & p,p1,r are_collinear & p1,p2,r1 are_collinear implies ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) )

assume that
A1: p,p1,p2 are_collinear and
A2: p,p1,r are_collinear and
A3: p1,p2,r1 are_collinear ; :: thesis: ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

A4: now :: thesis: ( p <> p2 implies ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) )
A5: now :: thesis: ( p1 <> p implies ex r, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) )
assume A6: p1 <> p ; :: thesis: ex r, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

take r = r; :: thesis: ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

A7: r,r1,r are_collinear by Def7;
p,p1,p are_collinear by Def7;
then p,p2,r are_collinear by A1, A2, A6, Def8;
hence ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) by A7; :: thesis: verum
end;
A8: now :: thesis: ( p1 <> p2 implies ex r1, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) )
assume A9: p1 <> p2 ; :: thesis: ex r1, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

take r1 = r1; :: thesis: ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

A10: r,r1,r1 are_collinear by Def7;
( p1,p2,p are_collinear & p1,p2,p2 are_collinear ) by ;
then p,p2,r1 are_collinear by A3, A9, Def8;
hence ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) by A10; :: thesis: verum
end;
assume p <> p2 ; :: thesis: ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

hence ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) by A5, A8; :: thesis: verum
end;
now :: thesis: ( p = p2 implies ex r, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) )
assume p = p2 ; :: thesis: ex r, r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

then A11: p,p2,r are_collinear by Def7;
take r = r; :: thesis: ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear )

r,r1,r are_collinear by Def7;
hence ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) by A11; :: thesis: verum
end;
hence ex r2 being Element of () st
( p,p2,r2 are_collinear & r,r1,r2 are_collinear ) by A4; :: thesis: verum