Include a union operation

[afandian]

Take the union of two or more existing artifacts to produce a third. Will be used to generate a master domain-list from, e.g.

• crossref-landing-page-domains
• datacite-landing-page-domains
• doi-resolver-domains
• => landing-page-domains

Also prefixes:

• crossref-doi-prefixes
• datacite-doi-prefixes
• => doi-prefixes

[afandian]

Also a disjoint.

e.g.

event-data-repo-tool merge --output domain-list --include crossref-domain-list --include datacite-domain-list --exclude exclude-domains