The second alignment is join 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'), r)
any pair which is in only one alignment is discarded.
Renders an alignment in JSON (and practically in JSON-LD)
http://json-ld.org/spec/latest/json-ld-syntax/
application/json <========= media type available