# description
If field is a list, how to manage the production.

# concept[].definition
when the target list is being assembled, the items for this rule go first. If more than one rule defines a first item (for a given instance of mapping) then this is an error.

# concept[].definition
the target instance is shared with the target instances generated by another rule (up to the first common n items, then create new ones).

# concept[].definition
when the target list is being assembled, the items for this rule go last. If more than one rule defines a last item (for a given instance of mapping) then this is an error.

# concept[].definition
re-use the first item in the list, and keep adding content to it.