docs: def key_leaf(self, data, schema, tree):
          """
          The deepest validation we can make in any given circumstance for a key.
          Does not recurse, it will just receive both values and the tree,
          passing them on to the :fun:`enforce` function.
          """
          key, value = data
          schema_key, schema_value = schema
          enforce(key, schema_key, tree, 'key')