Omit a fragment's mimetype when the fragment before it on the same node
already carried the same one.
Elision is per node id because fragments for different nodes interleave
freely inside one frame -- especially after the sender folds several messages
together, which is what makes this worth doing at all.
Returns the message unchanged when there was nothing to elide, so a frame that
cannot benefit allocates nothing and carries no marker.
Omit a fragment's mimetype when the fragment before it on the same node already carried the same one.
Elision is per node id because fragments for different nodes interleave freely inside one frame -- especially after the sender folds several messages together, which is what makes this worth doing at all.
Returns the message unchanged when there was nothing to elide, so a frame that cannot benefit allocates nothing and carries no marker.