This isn't really true. Even mature, sophisticated attempts at making hierarchical classification systems (e.g. the UDC[0]) don't claim to have created a syste

This isn't really true. Even mature, sophisticated attempts at making hierarchical classification systems (e.g. the UDC[0]) don't claim to have created a system where each document gets a unique leaf on a tree, and indeed instead attempt provide some syntax for adding additional metadata into its "nodes," i.e. the nodes become the sum of their tags. [0] https://en.wikipedia.org/wiki/Universal_Decimal_Classification https://en.wikipedia.org/wiki/Universal_Decimal_Classificati...