How do you raise and split ExceptionGroups manually?

Question presented to candidate: "Forget asyncio for a moment. Suppose you validate a form or process a batch, and you want to report every failure at once instead of stopping at the first one. How do you raise an ExceptionGroup yourself, and how do you take one apart with except*, split() and subgroup()?"

What a strong answer should cover:

• Raising: collect exceptions in a list and raise ExceptionGroup("message", errors); the list must be a non-empty sequence of exception instances; ExceptionGroup only accepts Exception subclasses, BaseExceptionGroup takes any and picks the right class (Python 3.11+, PEP 654).
• Catching: except* T handles the matching part of a group (every leaf, nesting kept); several except* clauses can each run once; unhandled parts and errors raised in handlers are re-raised as a group; a naked exception is wrapped.
• Splitting: eg.split(cond) returns (match, rest), eg.subgroup(cond) returns only the match; cond is a type, a tuple of types or a predicate (which also sees the groups themselves); no match gives None; nesting, __traceback__, __cause__, __context__ and __notes__ are kept.
• Subclasses: override derive() so split / subgroup rebuild your class with its extra fields; otherwise they produce plain ExceptionGroups.
• Rules: except and except* cannot mix in one try; except* ExceptionGroup is a TypeError; no break / continue / return in an except* block.

Code / implementation expected: Yes — raise a group of validation errors, catch it with except*, and show split() on a nested group.