// 正方形
@mixin square($size) {
  @include size($size, $size);
}

// size
@mixin size($width, $height) {
  width: $width;
  height: $height;
}

// placeholder
@mixin placeholder($color: #c5c8ce) {
  /*Firefox*/
  &::-moz-placeholder {
    color: $color;
    opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
  }

  /*Internet Explorer 10+*/
  &:-ms-input-placeholder {
    color: $color;
  }

  /*Safari and Chrome*/
  &::-webkit-input-placeholder {
    color: $color;
  }
}