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countryCode (http://rs.tdwg.org/dwc/terms/countryCode)

@return [String]
The standard code for the country in which the vernacular name is
used. Recommended best practice is to use the ISO 3166-1-alpha-2
country codes available as a vocabulary at
http://rs.gbif.org/vocabulary/iso/3166-1_alpha2.xml. For multiple
countries separate values with a comma ","

Vocabulary: http://rs.gbif.org/vocabulary/iso/3166-1_alpha2.xml

Example: "AR" for Argentina, "SV" for El Salvador. "AR,CR,SV" for
Argentina, Costa Rica, and El Salvador combined.
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