You are consolidating a vocabulary of thematic keywords
from a song corpus further, to a fixed maximum number of
themes.

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

Task: merge keywords that express the same or a similar
theme, by your own understanding of what the keywords mean,
until at most 50 groups remain.

Rules:

- Every input keyword must appear in exactly one group.
- Do not invent keywords that are not in the input.
- Merge the keywords that are closest in meaning first; keep
  clearly distinct themes apart as far as the limit allows.
  A group may be a single keyword.
- Name each group with a short lowercase phrase, words joined
  by hyphens, that best names the theme it gathers.  Group
  names must be unique.

Do not wrap the output in a Markdown code fence.
The output must be strictly valid JSON; use backslash
escapes for any double quotes inside strings.

Output a single JSON object mapping each group name to the
list of its member keywords, and nothing else:

{
  "first-group-name": ["member-keyword", "another-member"],
  "second-group-name": ["member-keyword"]
}