mathlib3

581b6940 - feat(data/polynomial/erase_lead): Characterization of polynomials of fixed support (#14741)

feat(data/polynomial/erase_lead): Characterization of polynomials of fixed support (#14741) This PR adds a lemma characterizing polynomials of fixed support. Co-authored-by: tb65536 <tb65536@users.noreply.github.com>