One of the contributors to the CMIP6 archive has contributed data with a 
`cell_methods` string of the form: `area: time: mean (interval: 1 month) where 
sea_ice`. CF Checker 4.0.0 reports this as an error `ERROR: (7.3): Invalid 
syntax for cell_methods attribute`. 

The relevant paragraph of the convention, the first in [sub-section 
7.3.2](http://cfconventions.org/cf-conventions/cf-conventions.html#recording-spacing-original-data),
 states that "To indicate more precisely how the cell method was applied, extra 
information may be included in parentheses () after the identification of the 
method." The author of the data file is interpreting the reference to "after 
the identification of the method" as meaning directly after `mean` in the 
`cell_methods` string. This is consistent with the usage of the word "method" 
throughout section 7.3. 

The conformance document, on the other hand, specifies that the the parentheses 
should come at the end of the phrase specifying the method, i.e.  `area: time: 
mean where sea_ice (interval: 1 month)`, and the CF Checker, as we expect, 
follows the conformance document.

My personal preference would be to follow the interpretation of the conformance 
document, provided this is consistent with the original intention of the 
sub-section in 7.3, but this would, I believe, require some clarification of 
the text, e.g. "To indicate more precisely how the cell method was applied, 
extra information may be included in parentheses () at the end of the phrase 
identifying the method."

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