0008 — Italian enza/enze: follow nltk, not the published description

Status: accepted · Date: 2026-08-05

Context

The published description of the Italian Snowball algorithm gives step 1 as:

enza enze → replace with ente if in R2

Implemented that way, esistenza becomes esistente, which step 3a trims to esistent.

nltk does something else:

elif suffix in ("enza", "enze"):
    word = suffix_replace(word, suffix, "te")

It substitutes te, not ente. So esistenza becomes esist + te = esistte, which step 3a trims to esistt. The same applies to differenza → differt.

The divergence only shows when the suffix is inside R2. potenza, pazienza, partenza and presenza fail that condition, fall through to step 3a, and stem to potenz, pazienz, partenz, presenz under either reading — which is why a small corpus can easily miss this.

Decision

Match nltk. ItalianSnowballStemmer replaces enza/enze with te.

The project's contract is behavioural parity with the Python libraries a user is migrating from, and for the stemmers that library is nltk — it is what the oracle corpora are frozen from and what docs/equivalence.md names as the reference. A stemmer that matched the prose but disagreed with the tool people actually use would be wrong in the only sense that matters here.

This is the same call as 0005, where jellyfish diverges from the textbook definition and we follow jellyfish.

Consequences

• esistenza and differenza stem to esistt and differt, which look wrong read as Italian. They are correct as parity, and that is the guarantee this library offers.
• If nltk ever aligns itself with the published description, the corpus will drift and the Oracles are reproducible job will catch it. At that point this decision should be revisited rather than the corpus quietly regenerated.
• Found because two of 96 corpus cases failed. The implementation had been written from the published description, and the reading looked right — the oracle is what turned a plausible misreading into a specific, locatable difference. Neither reasoning nor review would have caught it.