Add ParseRelative to Environment

environment.go

@@ -46,8 +46,25 @@ func (this *Environment) Init (ctx context.Context) error {
 // environment once parsed, and are returned on subsequent calls that have the
 // same name. 
 func (this *Environment) Parse (name string) (*Document, error) {
-	name = filepath.Clean(name)
-	input, err := this.GetFS().Open(name)
+	return this.ParseRelative(name, nil)
+}
+
+// ParseRelative is like Parse, but treats the name as a path relative to the
+// given document.
+func (this *Environment) ParseRelative (name string, document *Document) (*Document, error) {
+	var fileName string
+	if document == nil {
+		fileName = filepath.Clean(name)
+	} else {
+		nam, err := document.Abs(name)
+		if err != nil { return nil, err }
+		fileName = nam
+		nam, err = document.Rel(name)
+		if err != nil { return nil, err }
+		name = nam
+	}
+	
+	input, err := this.GetFS().Open(fileName)
 	if err != nil {
 		// if the file couldn't be opened, delete from cache in case the 
 		// file was deleted