bootstarp2.3.2 网格计算问题

gird > fluid 的布局部分有一行源码：.spanX (0) {}，不知道是何作用？

.fluid (@fluidGridColumnWidth, @fluidGridGutterWidth) {

    .spanX (@index) when (@index > 0) {
      .span@{index} { .span(@index); }
      .spanX(@index - 1);
    }

    // ?
    .spanX (0) {}

    .offsetX (@index) when (@index > 0) {
      .offset@{index} { .offset(@index); }
      .offset@{index}:first-child { .offsetFirstChild(@index); }
      .offsetX(@index - 1);
    }
    
    .offsetX (0) {}

    .offset (@columns) {
      margin-left: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1)) + (@fluidGridGutterWidth*2);
        *margin-left: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1)) - (.5 / @gridRowWidth * 100 * 1%) + (@fluidGridGutterWidth*2) - (.5 / @gridRowWidth * 100 * 1%);
    }

    .offsetFirstChild (@columns) {
      margin-left: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1)) + (@fluidGridGutterWidth);
      *margin-left: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1)) - (.5 / @gridRowWidth * 100 * 1%) + @fluidGridGutterWidth - (.5 / @gridRowWidth * 100 * 1%);
    }

    .span (@columns) {
      width: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1));
      *width: (@fluidGridColumnWidth * @columns) + (@fluidGridGutterWidth * (@columns - 1)) - (.5 / @gridRowWidth * 100 * 1%);
    }

    .row-fluid {
      width: 100%;
      .clearfix();
      [class*="span"] {
        .input-block-level();
        float: left;
        margin-left: @fluidGridGutterWidth;
        *margin-left: @fluidGridGutterWidth - (.5 / @gridRowWidth * 100 * 1%);
      }
      [class*="span"]:first-child {
        margin-left: 0;
      }

      // Space grid-sized controls properly if multiple per line
      .controls-row [class*="span"] + [class*="span"] {
        margin-left: @fluidGridGutterWidth;
      }

      // generate .spanX and .offsetX
      .spanX (@gridColumns);
      .offsetX (@gridColumns);
    }

  }

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刚定义了一个递归函数
那行就是调用递归