x only in taxon y if and only if x is in taxon y, and there is no other organism z such that y!=z a and x is in taxon z.

http://www.ncbi.nlm.nih.gov/pubmed/20973947

http://www.ncbi.nlm.nih.gov/pubmed/17921072

The original intent was to treat this as a macro that expands to 'in taxon' only ?Y - however, this is not necessary if we instead have supplemental axioms that state that each pair of sibling tax have a disjointness axiom using the 'in taxon' property - e.g. 'in taxon' some Eukaryota DisjointWith 'in taxon' some Eubacteria

lactation SubClassOf 'only in taxon' some 'Mammalia'

http://purl.obolibrary.org/obo/IAO_0000125

https://github.com/obophenotype/uberon/wiki/Taxon-constraints

Chris Mungall