decorator(caller) converts a caller function into a decorator;
    decorator(caller, func) decorates a function using a caller.

def decorator(caller, func=None):
    """
    decorator(caller) converts a caller function into a decorator;
    decorator(caller, func) decorates a function using a caller.
    """
    if func is None: # returns a decorator
        fun = FunctionMaker(caller)
        first_arg = inspect.getargspec(caller)[0][0]
        src = 'def %s(%s): return _call_(caller, %s)' % (
            caller.__name__, first_arg, first_arg)
        return fun.make(src, dict(caller=caller, _call_=decorator),
                        undecorated=caller)
    else: # returns a decorated function
        fun = FunctionMaker(func)
        src = """def %(name)s(%(signature)s):
    return _call_(_func_, %(signature)s)"""
        return fun.make(src, dict(_func_=func, _call_=caller), undecorated=func)