Consider changing the defaults of the group_span keyword of cf.Field.collapse to True (from None).

Consider changing the default of the group_contiguous keyword of cf.Field.collapse to 1 (from 0).

These are the defaults I originally would have liked when the keywords were introduced in v2.1.2 (https://ncas-cms.github.io/cf-python/Changelog.html), but I didn't use them to preserve backwards compatibility - i.e. the the default values chosen meant that a collapse at v2.1.1 would give the same result as one at v2.1.2.

The reason why the suggested new defaults are better is that they prevent, by default, values being returned that are created from a reduced sample size. Inspection of the bounds isn't guaranteed to expose this, and it is not obvious (although easy) to inspect the sample size of each group.