theorem Th95:
  o<>b & trap a,b,c,d,o implies not o,b,d are_collinear
proof
  assume that
A1: o<>b and
A2: trap a,b,c,d,o;
  o,a,b are_collinear by A2;
  then
A3: o,a // o,b;
  o,c,d are_collinear by A2;
  then
A4: o,c // o,d;
  o<>d & not o,a,c are_collinear by A1,A2,Th94;
  hence thesis by A1,A3,A4,Th23;
end;
