'Initialise with a Stack, the function name and the arguments.
All functions take the form of a single-item map in JSON::
{ <fn_name> : <args> }'

def __init__(self, stack, fn_name, args):

    super(Function, self).__init__()
    self._stackref = (weakref.ref(stack) if (stack is not None) else None)
    self.fn_name = fn_name
    self.args = args
