Does the cif or mmcif specification specify limits on the number of values an item may have?
For example, here's the cif definition of _struct.title. It's hard to see what sense there would ever be in a structure having more than one title but it seems that there's nothing limiting the number of titles.
Within a category, it would never make sense to have items with different numbers of values but again, I don't see anything in the format spec that would make such a thing strictly illegal even though it's nonsensical.
1 answer
CIF spec says:
A given data name (tag) (see 2.4 and 2.7) may appear no more than once in a given data block or save frame.
So any tag can have only one value in a block, unless it's in a loop.
Is there anything that prevents _struct.title from being defined in a loop?
I suppose it's specified in DDL2 which tags can be in loop, but I don't know details of DDL2.
In DDL1 dictionaries tags have attribute _list which says whether the defined item is to be declared in a looped list.
_list can have values yes, no or both and is no by default.
mmCIF dictionary is DDL2 and DDL1, but I guess something there says that _struct.title is not a list.
Edit: No, there is not. _struct.title must have a matching _struct.entry_id, which in turn should match _entry.id. The whole thing is clearly designed to allow for multiple entries in the same block (otherwise _struct.entry_id wouldn't be necessary), but this feature is probably never used.
(BTW you linked to mmcif spec at IUCR. For all practical purposes the PDBx/mmCIF spec at mmcif.pdb.org is more relevant)
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