diff --git a/addbatch_test.go b/addbatch_test.go
index 579c613..bbbb62f 100644
--- a/addbatch_test.go
+++ b/addbatch_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"math/big"
+	"os"
 	"runtime"
 	"sort"
 	"testing"
@@ -660,6 +661,7 @@ func benchAdd(t *testing.T, ks, vs [][]byte) {
 	c := qt.New(t)
 
 	dbDir := t.TempDir()
+	defer os.RemoveAll(dbDir) //nolint:errcheck
 	// storage, err := pebble.NewPebbleStorage(dbDir, false)
 	storage, err := leveldb.NewLevelDbStorage(dbDir, false)
 	c.Assert(err, qt.IsNil)
@@ -681,6 +683,8 @@ func benchAddBatch(t *testing.T, ks, vs [][]byte) {
 	c := qt.New(t)
 
 	dbDir := t.TempDir()
+	defer os.RemoveAll(dbDir) //nolint:errcheck
+	// storage, err := pebble.NewPebbleStorage(dbDir, false)
 	storage, err := leveldb.NewLevelDbStorage(dbDir, false)
 	c.Assert(err, qt.IsNil)
 	tree, err := NewTree(storage, 140, HashFunctionBlake2b)
diff --git a/tree.go b/tree.go
index d2be1ab..c19d231 100644
--- a/tree.go
+++ b/tree.go
@@ -278,23 +278,24 @@ func (t *Tree) down(newKey, currKey []byte, siblings [][]byte,
 
 	switch currValue[0] {
 	case PrefixValueEmpty: // empty
-		// TODO WIP WARNING should not be reached, as the 'if' above should avoid
-		// reaching this point
-		// return currKey, empty, siblings, nil
-		panic("should not be reached, as the 'if' above should avoid reaching this point") // TMP
+		fmt.Printf("newKey: %s, currKey: %s, currLvl: %d, currValue: %s\n",
+			hex.EncodeToString(newKey), hex.EncodeToString(currKey),
+			currLvl, hex.EncodeToString(currValue))
+		panic("This point should not be reached, as the 'if' above" +
+			" should avoid reaching this point. This panic is temporary" +
+			" for reporting purposes, will be deleted in future versions." +
+			" Please paste this log (including the previous lines) in a" +
+			" new issue: https://github.com/arnaucube/arbo/issues/new") // TMP
 	case PrefixValueLeaf: // leaf
-		if bytes.Equal(newKey, currKey) {
-			// TODO move this error msg to const & add test that
-			// checks that adding a repeated key this error is
-			// returned
-			return nil, nil, nil, ErrKeyAlreadyExists
-		}
-
 		if !bytes.Equal(currValue, emptyValue) {
 			if getLeaf {
 				return currKey, currValue, siblings, nil
 			}
 			oldLeafKey, _ := ReadLeafValue(currValue)
+			if bytes.Equal(newKey, oldLeafKey) {
+				return nil, nil, nil, ErrKeyAlreadyExists
+			}
+
 			oldLeafKeyFull := make([]byte, t.hashFunction.Len())
 			copy(oldLeafKeyFull[:], oldLeafKey)
 
@@ -385,6 +386,12 @@ func (t *Tree) newLeafValue(k, v []byte) ([]byte, []byte, error) {
 	return newLeafValue(t.hashFunction, k, v)
 }
 
+// newLeafValue takes a key & value from a leaf, and computes the leaf hash,
+// which is used as the leaf key. And the value is the concatenation of the
+// inputed key & value. The output of this function is used as key-value to
+// store the leaf in the DB.
+// [     1 byte   |     1 byte    | N bytes | M bytes ]
+// [ type of node | length of key |   key   |  value  ]
 func newLeafValue(hashFunc HashFunction, k, v []byte) ([]byte, []byte, error) {
 	leafKey, err := hashFunc.Hash(k, v, []byte{1})
 	if err != nil {
@@ -418,6 +425,12 @@ func (t *Tree) newIntermediate(l, r []byte) ([]byte, []byte, error) {
 	return newIntermediate(t.hashFunction, l, r)
 }
 
+// newIntermediate takes the left & right keys of a intermediate node, and
+// computes its hash. Returns the hash of the node, which is the node key, and a
+// byte array that contains the value (which contains the left & right child
+// keys) to store in the DB.
+// [     1 byte   |     1 byte    | N bytes  |  N bytes  ]
+// [ type of node | length of key | left key | right key ]
 func newIntermediate(hashFunc HashFunction, l, r []byte) ([]byte, []byte, error) {
 	b := make([]byte, PrefixValueLen+hashFunc.Len()*2)
 	b[0] = 2
diff --git a/tree_test.go b/tree_test.go
index 273b15b..6ecf112 100644
--- a/tree_test.go
+++ b/tree_test.go
@@ -141,12 +141,11 @@ func TestAddRepeatedIndex(t *testing.T) {
 	bLen := tree.HashFunction().Len()
 	k := BigIntToBytes(bLen, big.NewInt(int64(3)))
 	v := BigIntToBytes(bLen, big.NewInt(int64(12)))
-	if err := tree.Add(k, v); err != nil {
-		t.Fatal(err)
-	}
+
 	err = tree.Add(k, v)
-	c.Assert(err, qt.Not(qt.IsNil))
-	c.Check(err, qt.Equals, ErrMaxVirtualLevel)
+	c.Assert(err, qt.IsNil)
+	err = tree.Add(k, v) // repeating same key-value
+	c.Check(err, qt.Equals, ErrKeyAlreadyExists)
 }
 
 func TestUpdate(t *testing.T) {
diff --git a/vt.go b/vt.go
index 5d13f95..7ea8900 100644
--- a/vt.go
+++ b/vt.go
@@ -245,6 +245,7 @@ func (n *node) getNodesAtLevel(currLvl, l int) ([]*node, error) {
 	return nodes, nil
 }
 
+// upFromNodes builds the tree from the bottom to up
 func upFromNodes(ns []*node) (*node, error) {
 	if len(ns) == 1 {
 		return ns[0], nil
@@ -267,6 +268,7 @@ func upFromNodes(ns []*node) (*node, error) {
 	return upFromNodes(res)
 }
 
+// add adds a key&value as a leaf in the VirtualTree
 func (t *vt) add(fromLvl int, k, v []byte) error {
 	leaf := newLeafNode(t.params, k, v)
 	if t.root == nil {
@@ -282,7 +284,8 @@ func (t *vt) add(fromLvl int, k, v []byte) error {
 }
 
 // computeHashes should be called after all the vt.add is used, once all the
-// leafs are in the tree
+// leafs are in the tree. Computes the hashes of the tree, parallelizing in the
+// available CPUs.
 func (t *vt) computeHashes() ([][2][]byte, error) {
 	var err error
 
@@ -519,7 +522,8 @@ func flp2(n int) int {
 	return res
 }
 
-// returns an array of key-values to store in the db
+// computeHashes computes the hashes under the node from which is called the
+// method. Returns an array of key-values to store in the db
 func (n *node) computeHashes(currLvl, maxLvl int, p *params, pairs [][2][]byte) (
 	[][2][]byte, error) {
 	if n == nil || currLvl >= maxLvl {