The type of values contained in each document
Often when you are merging documents you will also need to clone them. Both
arguments to merge are frozen after the call so you can no longer call
mutating methods (such as change) on them. The symtom of this will be
an error which says "Attempting to change an out of date document". To
overcome this call clone on the argument before passing it to merge.
Merge
remoteintolocal