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[ddlm-group] CIF header

IMHO, there should be some sort of header to distinguish CIF variants, 
sort of like the DOCTYPE line at the top of XML files. This will help 
deal with CIF1 files that are not CIF2 compliant, and could also better 
handle more extreme variants, like binary CIF. The current syntax 
suggests, but does not require, an initial comment line starting with #CIF.

I proposed to the COMCIFS list that global_ could be used for this 
purpose. In this way, global_ would only be read by the parser, and not 
be considered as part of the actual CIF data. The idea is to use the 
existing STAR syntax instead of designing something new. The 
disadvantage is that the global_ section itself would have to maintain a 
restricted 7-bit ASCII format, and not allow any of the newer STAR/CIF 
syntax. So, the "simplicity" of just using the existing STAR syntax 
really is not there.

Alternatively, the initial CIF comment line could be made a requirement 
rather than a suggestion, and also define a way to include additional 
file attributes in the form of param=value pairs. For example, a CIF2 
file could add "binary=true" to indicate the presence of binary 
sections, rather than binary-CIF having to be a completely separate format.

If extra file attributes seem like an unnecessary complication, then 
maybe at least the simple comment line could be made a requirement? 
Then, you can distinguish CIF2 files, and assume that any file without a 
comment is CIF1.

Joe Krahn
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