"Round out tab" (based on http://css-tricks.com/tabs-with-round-out-borders/)

gistfile1.scss

// Tabs

@mixin round-out-tab($active_class: current, $radius: 8px, $tab_color: $purple, $outer_color: #fff) {
  position: relative;
  a {
    @extend .btn;
  }
  &.#{$active_class} {
    &:before,
    &:after {
      background: $tab_color;
      z-index: 1; // Below
      content: "";
      position: absolute;
      bottom: 0;
      width: $radius;
      height: $radius;
    }
    &:before {
      left: -$radius;
    }
    &:after {
      right: -$radius;
    }
    a {
      padding-bottom: $struc;
      border-radius: $radius $radius 0 0;
      &:before, &:after {
        background: $outer_color;
        z-index: 2; // Above
        width: $radius * 2;
        height: $radius * 2;
        border-radius: $radius;
        content: "";
        position: absolute;
        bottom: 0;
      }
      &:before {
        left: - $radius * 2;
      }
      &:after {
        right: - $radius * 2;
      }
    }
  }
}