1. Good idea to avoid spurious ambiguity by introducing head-initial constraint
   on first daughter of head-modifier-rule-post.

2. Unfortunate failure to parse one of the given test items:
   Happily the dog barked.
   Reason this fails to parse is because the head-modifier-rule-pre requires
   its head-dtr to have non-empty SPR.  Could fix this by removing this SPR
   constraint in the rule, and instead constraining the `adj' to require its
   MOD value to have a non-empty SPR (but not say this about `adv').

3. It seems that the constraint in the head-specifier-rule-post, that the
   SPR of the head-daughter be non-empty, is redundant, because that daughter
   has to be head-initial.