theorem Th37:
  ex X st a in X & b in X & c in X & X is being_plane
proof
  consider A such that
A1: a in A & b in A and
A2: A is being_line by Th11;
  ex X st c in X & A c= X & X is being_plane by A2,Th36;
  hence thesis by A1;
end;
