Mathlib: `A[X][X][X] ⧸ I` fails to find HasQuotient; add shortcut CommRing instances

In Mathlib (v4.35-era), example (I : Ideal ℤ[X][X][X]) : Type := ℤ[X][X][X] ⧸ I fails with 'failed to synthesize HasQuotient'. Depth two (A[X][X] ⧸ I) works. The trace shows Ideal.instHasQuotient_1 needs CommRing A[X][X][X], and the unifier fails CommRing A[X][X] =?= CommRing ?R[X] one level down. Fix: noncomputable local instance : CommRing A[X][X] := inferInstance (and one more per extra level). Give them names without underscores (e.g. commRingIterA2) and docstrings, or #lint flags defsWithUnderscore/docBlame. A abbrev P3 A := A[X][X][X] also works. A second gotcha: proving a lemma about A[X][X] ⧸ span {...} with (algebraMap B S).comp (Ideal.Quotient.mk I) fails with a Ring-vs-CommSemiring mismatch that mentions unexposed instNeg. State the lemma for a generic R[X] with R a variable and instantiate R := A[X] later; then it elaborates.