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AA is used in the construction "How big a dog was it?"

         |   |    |    |
        How big   a   dog was it        

The article "a" thus carries "Ds+ & {AA- & HA-}".  Adjectives
carry "AA+", disjoined with their A+ (used in prenominal
adjectives) and "Pa-" (used in predicative adjectives).

This construction uses the "EA-" on adjectives, which is
also used for modifying adverbs like "very" and for
questions like "How big is it". Similarly, the "EAh+" on "how"
is also used for ordinary adjectival questions like "How big
is it". "How" therefore carries "EAh+ & {HA+}". Since the HA+
is optional, there is a danger of the unwanted construction
"*How big dogs were they". This is prohibited in
post-processing: see "EAh".

Grammar Documentation Page.