Re: [ddlm-group] Second proposal to allow looping of 'Set'categories

Dear Colleagues,

  I am very puzzled about the concerns being raised.  Please bear with me.

  An experimental data CIF, whether it is a core CIF, a macromolecular CIF, a powder CIF is in general a container for all the information about a particular experiment.  Within that container we may have various tags and values.  In some cases, for that experiment, there is only one instance of a value being assigned to that tag, and it is convenient to use the a tag value pair to express that assignment.  In some cases, even though we a dealing with a single instance of an experiment, in order to describe that single experiment, we need to allow for multiple values to be assigned to the same tag, and CIF provides the loop_ construct to allow us to present those multiple values with a single presentation of the name of the tag.  So if there is one author we might say

  _audit_author "Peter Pan"

  _audit_author_address "Second to the right, and straight on till morning"

but if there were a second author we might say

loop_
    _audit_author

    _audit_author_address
  "Peter Pan" "Second to the right, and straight on till morning"

  "Winnie the Pooh" "Hundred Aker Wood"

Notice that we don't make this into two separate loops to avoid creating an ambiguity about which author lives at which address.

No information would have been gained or lost if we had only one author and had chosen to use a loop instead, not would any information have been gained or lost in the case of a single author if we had presented the name and address in two separate loops.

We have created rules about which tags belong together in the same loop, but the reality is that there are people who either do not know those rules or who choose to ignore them. I would suggest that, as long as the intended result is clear and unambiguous, it is not only unnecessary but pointlessly obstructionist to rigidiy enforce such rules.  Please like to create very flat CIFs for convenience in data harvesting.  People like to create databases spanning multiple sets of results.  People (including the IUCr publications office) like to extract particular sets of tags with their values and add "extra" tags we have not thought of.

They are going to do these things whether we like it or not.  Yes it is helpful if we provide recommended best practices and warnings about practices that seem to cause difficulty, but if CIF is to be a maximally useful container, a maximally useful framework we need to focus on beings as descriptive as possible and avoid being pointlessly prescriptive.

Which is a very, very long way of saying that I think we should allow looping of tags in all cases in which the intended meaning is unambiguous, and focus the most of attention of warnings in software on specific things in specific datasets that seem likely to result in ambiguous interpretions of those specific datasets.

I hope that made some sense.

Regards,

    Herbert