You are consolidating a vocabulary of thematic keywords that
were independently annotated on the songs of a corpus.

Input: a JSON array of thematic keywords, deduplicated and
unordered.

Task: merge keywords that express the same or nearly the same
theme, by your own understanding of what the keywords mean.

Rules:

- Every input keyword must appear in exactly one group.
- Do not invent keywords that are not in the input.
- Merge only what you consider the same theme; a group may be
  a single keyword.

Output a single JSON array of groups — each group an array of
its member keywords — and nothing else:

[
  ["keyword", "another-keyword"],
  ["keyword"]
]