Parameter syntax:

[ xml="namespace"|xml:prefix="namespace" 
  [ NEWLINE_CHAR xml="namespace"|xml:prefix="namespace" ]* ]?

Without any parameter, this command displays a modal dialog box (similar to the dialog displayed by Tools → Declare Namespace) which lets the user declare new name spaces and/or change the prefixes of existing name spaces.

The optional parameter specifies a namespace/prefix mapping and may be used to completely replace the current namespace/prefix mapping of the document being edited. However, in such case, this command will fail if the document is opened in read-only mode or if the document conforms to a DTD (the namespace/prefix mapping specified by a DTD is immutable).