note_creation.rs

 // create note & commitment
  let note = to
      .create_note(value, Fs::random(&mut rng), &JUBJUB)
      .unwrap();
  let cm = Node::new(note.cm(&JUBJUB).into_repr());

  // create empty commitment tree and add commitment
  let mut tree = CommitmentTree::new();
  tree.append(cm).unwrap();
  let witness = IncrementalWitness::from_tree(&tree);
  let merkle_path = witness.path().unwrap();

  let anchor = Fr::from(witness.root());

  let mut ctx = SaplingProvingContext::new();
  let alpha = Fs::random(&mut rng);

  let (proof, cv, rk) = SaplingProvingContext::spend_proof(
      &mut ctx,
      proof_generation_key, //proof_generation_key: ProofGenerationKey<Bls12>,
      *to.diversifier(),    //diversifier: Diversifier,
      note.r,               // rcm: Fs,
      alpha,                //ar: Fs,
      note.value,           //value: u64,
      anchor,               //anchor, rt: Fr,
      merkle_path,          //merkle_path: MerklePath<Node>,
      &params.spend_params, //proving_key: &Parameters<Bls12>,
      &params.spend_vk,     //verifying_key: &PreparedVerifyingKey<Bls12>,
      &JUBJUB,              //params: &JubjubBls12,
  )
  .expect("proving should not fail");