Extract 'headerHeight' and thus simplify 'headers'
Extract 'headerHeight' and thus simplify 'headers'

I think 'headers' could be written as a self-referential list, where the
y-coordinate of each element is a function of itself, because
(headerHeight . strs) is not strict in the 'shapey' field of Header. But
that would be silly.

wjt/bustle,wjt/bustle