def undeclared_namespace_bindings(ns_stack, ns_explicit)
      Hash[ns_explicit.map do |prefix,uri|
             [prefix, uri] if !find_namespace_uri(ns_stack, prefix, uri)
           end.compact]
    end
def undeclared_namespace_bindings(ns_stack, ns_explicit)
      Hash[ns_explicit.map do |prefix,uri|
             [prefix, uri] if !find_namespace_uri(ns_stack, prefix, uri)
           end.compact]
    end
figure out which explicit namespaces need declaring

 +ns_stack+ is the stack of namespace bindings
 +ns_explicit+ is the explicit refs for a element