'Lookup a function.
Args:
user_str: A raw string from the user, which may include uninterpreted
arguments.  For example, \'pluralize person people\' or \'test? admin\'
Returns:
A 2-tuple of (function, args)
function: Callable that formats data as a string
args: Extra arguments to be passed to the function at expansion time
Should be None to pass NO arguments, since it can pass a 0-tuple too.'

def Lookup(self, user_str):

    raise NotImplementedError
