diff --git a/api/filesys/filesystem.go b/api/filesys/filesystem.go
index 2d8caf2b8..e5034092b 100644
--- a/api/filesys/filesystem.go
+++ b/api/filesys/filesystem.go
@@ -28,6 +28,8 @@ type FileSystem interface {
 	Open(path string) (File, error)
 	// IsDir returns true if the path is a directory.
 	IsDir(path string) bool
+	// ReadDir returns a list of files and directories within a directory.
+	ReadDir(path string) ([]string, error)
 	// CleanedAbs converts the given path into a
 	// directory and a file name, where the directory
 	// is represented as a ConfirmedDir and all that implies.
diff --git a/api/filesys/fsnode.go b/api/filesys/fsnode.go
index 2593e705c..13b0a2298 100644
--- a/api/filesys/fsnode.go
+++ b/api/filesys/fsnode.go
@@ -349,6 +349,29 @@ func (n *fsNode) IsDir(path string) bool {
 	return result.isNodeADir()
 }
 
+// ReadDir implements FileSystem.
+func (n *fsNode) ReadDir(path string) ([]string, error) {
+	if !n.IsDir(path) {
+		return nil, fmt.Errorf("%s is not a directory", path)
+	}
+
+	dir, err := n.Find(path)
+	if err != nil {
+		return nil, err
+	}
+	if dir == nil {
+		return nil, fmt.Errorf("could not find directory %s", path)
+	}
+
+	keys := make([]string, len(dir.dir))
+	i := 0
+	for k := range dir.dir {
+		keys[i] = k
+		i++
+	}
+	return keys, nil
+}
+
 // Size returns the size of the node.
 func (n *fsNode) Size() int64 {
 	if n.isNodeADir() {
diff --git a/api/filesys/fsondisk.go b/api/filesys/fsondisk.go
index 6becbb6f1..86cd26d75 100644
--- a/api/filesys/fsondisk.go
+++ b/api/filesys/fsondisk.go
@@ -100,6 +100,19 @@ func (fsOnDisk) IsDir(name string) bool {
 	return info.IsDir()
 }
 
+// ReadDir delegates to os.ReadDir
+func (fsOnDisk) ReadDir(name string) ([]string, error) {
+	dirEntries, err := os.ReadDir(name)
+	if err != nil {
+		return nil, err
+	}
+	result := make([]string, len(dirEntries))
+	for i := range dirEntries {
+		result[i] = dirEntries[i].Name()
+	}
+	return result, nil
+}
+
 // ReadFile delegates to ioutil.ReadFile.
 func (fsOnDisk) ReadFile(name string) ([]byte, error) { return ioutil.ReadFile(name) }
 
diff --git a/api/types/replacement.go b/api/types/replacement.go
index 46618b68b..1f59239d9 100644
--- a/api/types/replacement.go
+++ b/api/types/replacement.go
@@ -28,10 +28,10 @@ type SourceSelector struct {
 	resid.ResId `json:",inline,omitempty" yaml:",inline,omitempty"`
 
 	// Structured field path expected in the allowed object.
-	FieldPath string `json:"fieldPath" yaml:"fieldPath"`
+	FieldPath string `json:"fieldPath,omitempty" yaml:"fieldPath,omitempty"`
 
 	// Used to refine the interpretation of the field.
-	Options *FieldOptions `json:"options" yaml:"options"`
+	Options *FieldOptions `json:"options,omitempty" yaml:"options,omitempty"`
 }
 
 func (s *SourceSelector) String() string {
@@ -54,34 +54,34 @@ type TargetSelector struct {
 	Select *Selector `json:"select" yaml:"select"`
 
 	// From the allowed set, remove objects that match this.
-	Reject []*Selector `json:"reject" yaml:"reject"`
+	Reject []*Selector `json:"reject,omitempty" yaml:"reject,omitempty"`
 
 	// Structured field paths expected in each allowed object.
-	FieldPaths []string `json:"fieldPaths" yaml:"fieldPaths"`
+	FieldPaths []string `json:"fieldPaths,omitempty" yaml:"fieldPaths,omitempty"`
 
 	// Used to refine the interpretation of the field.
-	Options *FieldOptions `json:"options" yaml:"options"`
+	Options *FieldOptions `json:"options,omitempty" yaml:"options,omitempty"`
 }
 
 // FieldOptions refine the interpretation of FieldPaths.
 type FieldOptions struct {
 	// Used to split/join the field.
-	Delimiter string `json:"delimiter" yaml:"delimiter"`
+	Delimiter string `json:"delimiter,omitempty" yaml:"delimiter,omitempty"`
 
 	// Which position in the split to consider.
-	Index int `json:"index" yaml:"index"`
+	Index int `json:"index,omitempty" yaml:"index,omitempty"`
 
 	// TODO (#3492): Implement use of this option
 	// None, Base64, URL, Hex, etc
-	Encoding string `json:"encoding" yaml:"encoding"`
+	Encoding string `json:"encoding,omitempty" yaml:"encoding,omitempty"`
 
 	// If field missing, add it.
-	Create bool `json:"create" yaml:"create"`
+	Create bool `json:"create,omitempty" yaml:"create,omitempty"`
 }
 
 func (fo *FieldOptions) String() string {
-	if fo == nil || fo.Delimiter == "" {
+	if fo == nil || (fo.Delimiter == "" && !fo.Create) {
 		return ""
 	}
-	return fmt.Sprintf("%s(%d)", fo.Delimiter, fo.Index)
+	return fmt.Sprintf("%s(%d), create=%t", fo.Delimiter, fo.Index, fo.Create)
 }
diff --git a/api/types/selector.go b/api/types/selector.go
index 4ad1eb130..2c07f0b01 100644
--- a/api/types/selector.go
+++ b/api/types/selector.go
@@ -28,6 +28,10 @@ type Selector struct {
 	LabelSelector string `json:"labelSelector,omitempty" yaml:"labelSelector,omitempty"`
 }
 
+func (s *Selector) Copy() Selector {
+	return *s
+}
+
 func (s *Selector) String() string {
 	return fmt.Sprintf(
 		"%s:a=%s:l=%s", s.ResId, s.AnnotationSelector, s.LabelSelector)