Flatten nested map into rows

Question

In clojure, how can I turn a nested map like this:

{"1" {"1.1" {}
      "1.2" {}}
 "2" {"2.1" {}
      "2.2" {}
      "2.3" {}}}

Into this:

[["1" "1.1"]
 ["1" "1.2"]
 ["2" "2.1"]
 ["2" "2.2"]
 ["2" "2.3"]]

Provided that nesting is not limited