Clarify behavior of traverse on an absent field.

It should return an error explicitly.

A pair of "(nil, nil)" reponses would also be an unambiguous option for
representing an absent member, but seems more likely to create pitfalls
than save a significant amount of code for callers.
Signed-off-by: Eric Myhre <hash@exultant.us>

impl/free/freeNode.go

@@ -30,7 +30,6 @@ type Node struct {
 	kind ipld.ReprKind
 
 	_map   map[string]ipld.Node // Value union.  Only one of these has meaning, depending on the value of 'Type'.
-	_map2  map[int]ipld.Node    // Value union.  Only one of these has meaning, depending on the value of 'Type'.
 	_arr   []ipld.Node          // Value union.  Only one of these has meaning, depending on the value of 'Type'.
 	_bool  bool                 // Value union.  Only one of these has meaning, depending on the value of 'Type'.
 	_int   int                  // Value union.  Only one of these has meaning, depending on the value of 'Type'.
@@ -78,20 +77,11 @@ func (n *Node) TraverseField(pth string) (ipld.Node, error) {
 	case ipld.ReprKind_Null:
 		return nil, fmt.Errorf("cannot traverse terminals")
 	case ipld.ReprKind_Map:
-		switch {
-		case n._map != nil:
-			v, _ := n._map[pth]
-			return v, nil
-		case n._map2 != nil:
-			i, err := strconv.Atoi(pth)
-			if err != nil {
-				return nil, fmt.Errorf("404")
-			}
-			v, _ := n._map2[i]
-			return v, nil
-		default:
-			panic("unreachable")
+		v, exists := n._map[pth]
+		if !exists {
+			return nil, fmt.Errorf("404")
 		}
+		return v, nil
 	case ipld.ReprKind_List:
 		i, err := strconv.Atoi(pth)
 		if err != nil {

node.go

@@ -15,12 +15,22 @@ type Node interface {
 	// GetField resolves a path in the the object and returns
 	// either a primitive (e.g. string, int, etc), a link (type CID),
 	// or another Node.
-	TraverseField(path string) (Node, error)
+	//
+	// If the Kind of this Node is not ReprKind_Map, a nil node and an error
+	// will be returned.
+	//
+	// If the key does not exist, a nil node and an error will be returned.
+	TraverseField(key string) (Node, error)
 
 	// GetIndex is the equivalent of GetField but for indexing into an array
 	// (or a numerically-keyed map).  Like GetField, it returns
 	// either a primitive (e.g. string, int, etc), a link (type CID),
 	// or another Node.
+	//
+	// If the Kind of this Node is not ReprKind_List, a nil node and an error
+	// will be returned.
+	//
+	// If idx is out of range, a nil node and an error will be returned.
 	TraverseIndex(idx int) (Node, error)
 
 	// Keys returns instructions for traversing the node.