The second alignment is composed with the first one meaning that for
any pair (o, o', n, r) in O and (o',o", n', r') in O' the resulting
alignment will contain:
( o, o", join(n,n'), compose(r, r')) iff compose(r,r') exists.
Cut refinement :
- above n (hard)
- above n under the best ()
- getting the n% better (perc)
- getting the under n% of the best (prop)
- getting the n best values