The success of the generation strategy that is used by BUG very much depends on the definition of semantic head of a rule. The semantic heads are used to make it possible to compute links between a feature structure and a seed. Furthermore, by using these links, it is assumed that the semantics of other daughters of a rule get instantiated, to make a directed generation process possible. Therefore, the following condition <11> on semantic heads applies.
The first part of this condition states that the semantics of
the semantic head of a rule must be predictable from the
semantics of the mother node of this rule.
The second part of <11> states that the semantics of the
daughters of a rule must be predictable from the semantics
of the mother and the syntax of the semantic head.
The first part of the condition is a condition on rules, whereas
the second part of the condition is a condition on grammars,
because it is not clear what the syntax of the head
can turn out to be, without looking at the complete grammar.