old_contents: import _ from 'lodash'
  
  export default function() {
    return function({ addUtilities, theme, variants }) {
      const utilities = _.fromPairs(
        _.map(theme('zIndex'), (value, modifier) => {
          return [
            `.z-${modifier}`,
            {
              'z-index': value,
            },
          ]
        })
      )
  
      addUtilities(utilities, variants('zIndex'))
    }
  }
  
new_contents: import _ from 'lodash'
  import prefixNegativeModifiers from '../util/prefixNegativeModifiers'
  
  export default function() {
    return function({ addUtilities, e, theme, variants }) {
      const utilities = _.fromPairs(
        _.map(theme('zIndex'), (value, modifier) => {
          return [
            `.${e(prefixNegativeModifiers('z', modifier))}`,
            {
              'z-index': value,
            },
          ]
        })
      )
  
      addUtilities(utilities, variants('zIndex'))
    }
  }
  
subject: Add support for negative prefixes in zIndex plugin