':returns: a reference to the value that was set via `set_value_once`
:raises AssertionError: if result was not set or value is incompatible
with `set_value_once`'

@property
def value(self):

    assert self._ready, '_CallbackResult    was    not    set'
    assert (isinstance(self._values, tuple) and (len(self._values) == 1)), ('_CallbackResult    value    is    incompatible    with    set_value_once:    %r' % (self._values,))
    return self._values[0]
