Overview
Slots, or archetype slots are statements within archetypes that other archetypes may (or must) be included at this point in the composition. They are constraint statements about what might be sensible to add at that particular point. Clearly the reference model itself is constrained - sections can only contain other sections or entries, tree structures can only contain clusters or elements and lists only elements. Slots go beyond these limits to say what archetypes are sensible to add at this point - but the archetypes must be of the appropriate sort (so a cluster element is only allowed to be added within an entry for example).
NOTE: most of the problems described below are now dealt with in detail here.
Multiple slots - how to represent them?
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