code: function flipShadow(value) {
    var elements = value.split(/\s+/);
  
    if (!elements) { return value; }
  
    var inset = (elements[0] == 'inset') ? elements.shift() + ' ' : '';
    var property = elements[0].match(/^([-+]?\d+)(\w*)$/);
  
    if (!property) { return value; }
  
    return inset + [(-1 * +property[1]) + property[2]]
        .concat(elements.splice(1)).join(' ');
  }
docs: Flip a single `box-shadow` value.
  
  @param {String} value Value
  @return {String}