# File htree/context.rb, line 37
    def subst_namespaces(declared_namespaces)
      namespaces = @namespaces.dup
      declared_namespaces.each {|k, v|
        check_namespace_prefix(k)
        check_namespace_uri(v)
        namespaces[k] = v
      }
      Context.new(namespaces)
    end