message: Add TODO for improving lookup performance.
diff: @@ -131,6 +131,10 @@ fn lookup3_xy_with_conditional_negation<E: Engine, CS, Var: Copy>(
  ) -> Result<(AllocatedNum<E, Var>, AllocatedNum<E, Var>), SynthesisError>
  where CS: ConstraintSystem<E, Variable=Var>
  {
  + // TODO: This can be made into a 2-constraint lookup
  + // if it can return linear combinations rather than
  + // allocated numbers.
  +
  assert_eq!(bits.len(), 3);
  assert_eq!(coords.len(), 4);
  