Portuguese non-headed sentences

[José Catarino]

I want to thank the suggestions about sentences like
(1) Chuva, só no sábado
'Rain, only Saturday'
This sentences are (relatively) context-free, meaningfully and well-formed and
they aren't an ellipsis instance. Thus, they aren't present only in slogans
and newspaper's titles, but they can occur in many situations:
(2) a. Que tempo!
'What a whether!'
(2) b. Está descansado. Chuva, só no sábado.
'Don't worry. It rains only Saturday'.
So, I will try to describe expressions like (1) and (2.b) as non-head
sentences.
I think (but I am not sure) that if these sentences are, indeed, context-free
and non-headed, they may pose some problems to
the HPSG grammar (eg. the Head Feature Principle, the Valence Principle,
The Semantic Compositionality  Principle, by example.)
Is this point of vue raisonable?
Thanks.
José Catarino