A problem for BUG is posed by rules where a daughter (that is not the semantic head) has the same semantics as the mother node. An example is rule <12>
In this type of rules, that usually occur in the case of auxiliaries and function words, BUG will not terminate, because after the construction of H it will try to construct the Comp. However this process starts with the same goal as the construction of sbar. Therefore this same rule will apply again and again. I hypothesize that this only occurs in the case of function words, case markers and things like that. In this case the syntactic head often differs from the semantic head; moreover the syntactic head is a lexical entry. Therefore I add <11c> to <11>.
BUG is modified so that it recognizes cases of syntactic heads. In that case
only lexical lookup is possible, and BUG will terminate for grammars that obey
<11> (except for grammars that use rules like <1>).