theorem Th11:
  P = Dir u & qfconic(a,b,c,d,e,f,u) = 0 implies
  P in conic(a,b,c,d,e,f)
  proof
    assume that
A2: P = Dir u and
A3: qfconic(a,b,c,d,e,f,u) = 0;
    for v be Element of TOP-REAL 3 st v is non zero & P = Dir v holds
    qfconic(a,b,c,d,e,f,v) = 0 by A2,A3,Th10;
    hence thesis;
  end;
