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')