Different rules for expansion and merging
In the first iterations of the specification draft, the merge and expansion rules are described as two separate entities : the merge rules are used to merge multiple input documents, while the expansion rules are used to transform merged values. Note that expansion rules may also define transformations that merge multiple values together, and such rules should be able to refer to the existing merge rules.
Because of this, expansion and merge rules should probably be merged together, representing merge rules as expansion rules.
This issue should be used to discuss this change and its implications,