let S be IncSpace; for A being POINT of S holds
not for B, C, D being POINT of S holds {A,B,C,D} is planar
let A be POINT of S; not for B, C, D being POINT of S holds {A,B,C,D} is planar
set L = the LINE of S;
consider B being POINT of S such that
A1:
A <> B
and
B on the LINE of S
by Lm1;
not for C, D being POINT of S holds {A,B,C,D} is planar
by A1, Th47;
hence
not for B, C, D being POINT of S holds {A,B,C,D} is planar
; verum