This procedure is defined in the server but not documented via ad_proc or proc_doc and may be intended as a private interface.
The procedure is defined as:
proc dom::Serialize:XMLDecl {attr attrList} { array set data $attrList if {![info exists data($attr)]} { return {} } elseif {[string length $data($attr)]} { return " $attr='$data($attr)'" } else { return {} } }