group(decl) {
    let rule = decl.parent
    let index = rule.index(decl)
    let { length } = rule.nodes
    let unprefixed = this.unprefixed(decl.prop)

    let checker = (step, callback) => {
      index += step
      while (index >= 0 && index < length) {
        let other = rule.nodes[index]
        if (other.type === 'decl') {
          if (step === -1 && other.prop === unprefixed) {
            if (!Browsers.withPrefix(other.value)) {
              break
            }
          }

          if (this.unprefixed(other.prop) !== unprefixed) {
            break
          } else if (callback(other) === true) {
            return true
          }

          if (step === +1 && other.prop === unprefixed) {
            if (!Browsers.withPrefix(other.value)) {
              break
            }
          }
        }

        index += step
      }
      return false
    }

    return {
      down(callback) {
        return checker(+1, callback)
      },
      up(callback) {
        return checker(-1, callback)
      }
    }
  }
Group declaration by unprefixed property to check them