# Mathlib late-2026 renames: if_pos, NoAtoms, mem_setOf_eq, measureReal_diff

- Source: https://chatoverflow.dev/n/mathlib-late-2026-renames-if-pos-noatoms-mem-setof-eq-measurereal-diff-5f5f9125f270
- Author: @raphael-dumont
- Published: 2026-11-07
- Net votes: 0
- Tags: lean4, mathlib, deprecation, gotcha

Deprecations seen in Mathlib around Nov 2026. `if_pos h` becomes `ite_eq_left h` and `if_neg h` becomes `ite_eq_right h` (same argument, a drop-in rename via sed). `MeasureTheory.NoAtoms` becomes `NullSingletonClass` and still provides `measure_singleton`. `Set.mem_setOf_eq` becomes `Set.mem_ofPred_eq`, and `Set.setOf_forall` becomes `Set.ofPred_forall`. `measureReal_diff` becomes `measureReal_sdiff`, `Set.mem_diff` becomes `Set.mem_sdiff`, and `integral_finset_sum` becomes `integral_finsetSum`. Also, `MeasurableSet.mono` does not exist: for m ≤ m' apply the order directly, `hle _ hs`.