Tags: hierarchy, equivalence, and (maybe) correlation

I would like to propose the implementation of tag hierarchy, tag equivalence, and maybe tag correlation, as follows:

1. Tag hierarchy:

1.1. Tags would be nestable.

• The tags "Germany", "France", and other European countries' names could be all nested under "Europe". As result, searching for "Europe" would bring anything tagged under "France" or "Germany" or simply "Europe".
• Subjects as "federal public administration" will be under "public administration".
• Organizations such as "ECHR" (European Court of Human Rights) will be under "international courts".

1.2. Tags would be nestable in multiple levels.

• "Berlin" could be nested under "Germany", which would be nested under "Europe".
• "federal public administration" will be under "public administration", which will be under "administration".
• "ECHR" (European Court of Human Rights) will be under "international courts", which will be under "judicial courts".
  

1.3. Tags would be nestable under more than one superior tag.

• "Berlin" would be nested under "Germany" but also under "capital cities".
• "Germany" would be under "Europe" but also under "European Union" and "Council of Europe". This is relevant because "Switzerland" will be under "Europe" and "Council of Europe" only, and will not be considered when the tag "European Union" is used.
• "federal public administration" will be under both "public administration" and "federalism".
• "ECHR" (European Court of Human Rights) will be under both "international courts", "Council of Europe", "human rights courts" (which will be under both "judicial courts" and "human rights").

While the above are not available, one has to manually add all applicable tags to, e.g., an item regarding a (hypothetical) decision by the ECHR on a case in which Germany's federal public administration was accused of omission in local level implementation of a given human right for children: "Europe", "European Union", "Council of Europe", "Germany", "federal public administration", "public administration", "ECHR", and many, many others.

2. Tag equivalence:

This would help inputting tags, and also making sense of tags automatically imported, e.g.:

• federalism = federalismo = federalisme = federação = federation
• law = direito = derecho = droit
• constitutional law = droit constitutionnel = derecho constitucional = direito constitucional

This is useful both for tags with same meaning in multiple languages and for differently worded tags with identical (or close enough) meaning (such as "federalism" and "federation").

3. Tag correlation:

Perhaps a third useful way of connecting tags would be a softer one: correlation. E.g.: "morality" ~ "honesty"; "confidentiality" ~ "secret". This could be implemented in an "opt-in" or "opt-out" way.

In the opt-in way, searches including "morality" would not include items tagged with "honesty" (and not tagged with "morality", otherwise they would naturally be included, of course), unless the user used some kind of search operator modifying the tag ("tag~", as opposed to "tag", for instance, or any other).

The opposite would occur in the opt-out: items tagged with "honesty" would be automatically included in searches including tag "morality", unless something like "tag!" was used instead of "tag".

This would be useful because in one research context we might see a difference between two tags that later is no longer relevant.