cannot declare method that has only one lazy argument

@yaql.specs.method
@yaql.specs.parameter('expression', yaql.yaqltypes.Lambda())
def some_method(expression, context):
   pass

context = yaql.create_context()
context.register_function(some_method) # raises Exception

The following exception occurs:
Traceback (most recent call last):
    context.register_function(changed)
  File "/Users/bgaifullin/py2.7/lib/python2.7/site-packages/yaql/language/contexts.py", line 126, in register_function
    raise exceptions.InvalidMethodException(spec.name)
yaql.language.exceptions.InvalidMethodException: Function 'changed' cannot be called as a method