 empty variable, entity
and relation type sets. There is an <span style="font-style: italic;">initial
</span>theory whose base is the
initial language and whose axiom set is empty. Any two languages have a
<span style="font-style: italic;">sum </span>defined via the disjoint
unions of variable, entity and relation
type sets. Any two theories have a sum defined as the sum of their base
type languages and the union of their axiom sets. A span over a theory
pair is a third theory that is the source of two theo