input.stylus

.container {  
  btn(width) {
    display: flex;
    flex-direction: column;
    width: width;
    height: 50px;
    
    ../ button^[1..-1] {
      display: inline-block;
    }
  }
  
  .btn {
    btn(100px);
  }
}

output.css

.container .btn {
  display: flex;
  flex-direction: column;
  width: 100px;
  height: 50px;
}
.container button.btn {
  display: inline-block;
}