On 6 April 2011, Jonathan Rochkind wrote:

> I think we computer programmers are really better-served by reserving the 
> notion of "validity" for things specified by formal specifications -- as we 
> normally do, talking about any other data format.   And the only formal 
> specifications I can find for Marc21 say that leader bytes 20-23 should be 
> 4500. (Not true of Marc in general just Marc21).

"Validity" does mean something definite ... but Postel's Law is a good 
guideline, especially with the swamp of bad MARC, old MARC, alternate 
MARC, that's out there.  Valid MARC is valid MARC, but if---for the sake 
of file and its magic---we can identify technically invalid but still 
usable MARC, that's good.

Bill
-- 
William Denton, Toronto : miskatonic.org www.frbr.org openfrbr.org