[36taransingh5-dotcom]

Problem

After a PSet evolver is changed and materialized, calling persistent() again returns the original set instead of the latest snapshot:

from pyrsistent import pset

evolver = pset([1]).evolver()
evolver.add(2)
assert evolver.persistent() == pset([1, 2])
assert evolver.persistent() == pset([1, 2])  # fails on master: returns pset([1])

Materializing the underlying map clears its dirty state, but the set evolver's cached _original_pset still refers to the old set.

Change

Update the cached set when materializing a dirty evolver, then return that snapshot on subsequent calls. This follows the existing PMap evolver pattern. Add four regression cases covering add/remove snapshots, a no-op after a snapshot, and sequential snapshots that remain independent.

Validation

On Linux, Python 3.12.15:

• The four new regression cases fail before the fix.
• Full suite with the C extension: 641 passed, 1 skipped.
• Full suite with PYRSISTENT_NO_C_EXTENSION=1: 540 passed, 102 skipped (backend-specific tests).
• Module doctests: 80 passed.
• Source distribution and wheel build succeeded.
• git diff --check passed.

The published source and tests were read back and the suites rerun against those contents. This fix was found by source inspection and local reproduction, not a reported production incident.

AI assistance was used to investigate, implement, and test this contribution.