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Re: Permitting new physical units?

Hi John, sorry for the delayed response to your contribution to
the _refine_diff_density_* units discussions. Opportunities to 
look at this stuff is rather limited at the moment.

>  Although it is not stated in the new DDL definition,
> I assume that any one of these strings can then be used as a suffix
> to the root data item name producing a complete data item name
> including the specification of units.

The use of units suffixes was allowed in the original DDL1 but was
abandoned several years ago as too difficult to search against. I
believe that the advantages of the current _units_construct
suggestion is that the units specification (and definition) are
separated from the class of data items which have values dependent
on these units. This simplifies considerably the identification
of the item and its subsequent parsing.

> If on the other hand, you would want to be more creative in
> the construction of data item names, where unit information might
> be embedded anywhere within the data item name, then you
> would certainly have to move in the direction of the
> regex syntax that Syd has proposed.   If this is the
> case, then I think that the _units_construct DDL definition should
> be modified to define the construction for entire data item
> name, rather than the just the portion dealing with units.
Far from wishing to be more creative in the construction of data 
names I favour (and am advising) that we be more conservative...
and would be distinctly nervous about the suggestion in your last
sentence. I am advocating that the name of a data item be quite 
independent of the selected units... so that basically the same
names will (for items under discussion) be used for x-ray, neutron
and electron diffraction studies. In fact (and I hesitated saying
this before for fear of making it sound too complicated) the 
specification of _diffrn_radiation_probe could, by using conditional
branching in the _units_construct definition string be, used to 
directly specify the units... since there is 1:1 mapping of the 
probe string and the units string. One does not need to do this
but it is quite easily defined and could be readily understood
by non-experts.

Cheers, Syd.

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