docs: def value_leaf(self, data, schema, tree):
          """
          The deepest validation we can make in any given circumstance for
          a value. 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
  
          if hasattr(schema_value, '__validator_leaf__'):
              return schema_value(value, tree)
          enforce(value, schema_value, tree, 'value')