Semantics of AtTimeLinks

I created an agent to test the existing implementation of the [TemporalRule class](https://github.com/opencog/opencog/blob/master/opencog/python/pln/rules/temporal_rules.py#L38) in order to be able to modify it. As it uses [AtTimeLinks](http://wiki.opencog.org/w/Temporal_Reasoning#AtTimeLink), I would like to understand their semantics. I've loaded the (disabled) [temporalToyExample](https://github.com/opencog/opencog/blob/4efea046d295ba8422ec82513ff8f18e55e37144/tests/python/test_pln/scm_disabled/temporal/temporalToyExample.scm). When I run the example, I get the following error:
```
In unknown file:
   ?: 2 [memoize-variable-access! #<memoized AtTimeLink> #<directory # a2ac6c0>]
In ice-9/boot-9.scm:
 102: 1 [#<procedure b03f700 at ice-9/boot-9.scm:97:6 (thrown-k . args)> unbound-variable ...]
In unknown file:
   ?: 0 [catch-closure unbound-variable #f "Unbound variable: ~S" (AtTimeLink) #f]
 
ERROR: In procedure catch-closure:
ERROR: Unbound variable: AtTimeLink
ABORT: unbound-variable
```
Other link types don't cause this error and AtTimeLinks is number 55 in the link types file. Does anyone know why this error occurs? @cosmoharrigan? @linas?