Added more error handling and necessary functions

source.go

@@ -23,6 +23,11 @@ type Source struct {
 	Extensions   []*ExtensionElement `xml:",any,omitempty"`
 }
 
+// NewSource creates a new Source. It returns a *Source.
+func NewSource() *Source {
+	return new(Source)
+}
+
 // Check checks the Source for incompatibilities with RFC4287. It returns an
 // error.
 func (s *Source) Check() error {