code: function intersect(a, b, not) {
    return a.filter((c) => {
      const index = ~indexOfDeclaration(b, c);
      return not ? !index : index;
    });
  }
docs: Returns filtered array of matched or unmatched declarations
  @param {postcss.Declaration[]} a
  @param {postcss.Declaration[]} b
  @param {boolean} [not=false]
  @return {postcss.Declaration[]}