Generated by SassMeister.com.

SassMeister-input-HTML.haml

%nav.vertical
  %h1.header.vertical--first header
  %ul.list.vertical--last
    %li.item
      %a.link blah
    - for i in 1..2 do
      %li.item__nested
        %a.link nested!
        %ul.list
          %li.item
            %a.link inside
          %li.item
            %a.link inside 2
    %li.item__nested.vertical--last
      %a.link nested last
      %ul.list.vertical--last
        %li.item.vertical--last
          %a.link inside

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

$border-radius: 20px;
.vertical {
  border: 1px solid red;
  border-radius: $border-radius;
  width: 300px;
}
  
// State?
.vertical--first {
  box-shadow: inset 0 2px 0 0 rgba(255, 255, 255, 0.95);
  border: {
    top-left-radius: $border-radius;
    top-right-radius: $border-radius;
  }
}

.vertical--last {
  border: {
    bottom-left-radius: $border-radius;
    bottom-right-radius: $border-radius;
  }
}

.header {
  background: rgb(255, 100, 255);
  margin: 0;
  padding: 6px;
  
  // Never last
  border-bottom: 1px green solid;
}

.list {
  background: gray;
  margin: 0;
  list-style-type: none;
  padding-left: 0;
}

.item {
  ~ .item {
    border-top: 1px solid green;
  }
}

.link {
  display: inline-block;
  padding: 6px;
}
.item__nested {
  @extend .item;
  
  .list {
    background: rgb(200, 255, 255);
  }
  
  .item {
    .link {
      padding-left: 20px;
    }
    
    & ~ .item {
      border-top: 1px red solid;
    }
    
    &:first-child {
      border-top: 1px green solid;
    }
  }
}

SassMeister-output.css

.vertical {
  border: 1px solid red;
  border-radius: 20px;
  width: 300px;
}

.vertical--first {
  box-shadow: inset 0 2px 0 0 rgba(255, 255, 255, 0.95);
  border-top-left-radius: 20px;
  border-top-right-radius: 20px;
}

.vertical--last {
  border-bottom-left-radius: 20px;
  border-bottom-right-radius: 20px;
}

.header {
  background: #ff64ff;
  margin: 0;
  padding: 6px;
  border-bottom: 1px green solid;
}

.list {
  background: gray;
  margin: 0;
  list-style-type: none;
  padding-left: 0;
}

.item ~ .item, .item__nested ~ .item, .item ~ .item__nested, .item__nested ~ .item__nested {
  border-top: 1px solid green;
}

.link {
  display: inline-block;
  padding: 6px;
}

.item__nested .list {
  background: #c8ffff;
}
.item__nested .item .link, .item__nested .item__nested .link {
  padding-left: 20px;
}
.item__nested .item ~ .item, .item__nested .item__nested ~ .item, .item__nested .item ~ .item__nested, .item__nested .item__nested ~ .item__nested {
  border-top: 1px red solid;
}
.item__nested .item:first-child, .item__nested .item__nested:first-child {
  border-top: 1px green solid;
}

SassMeister-rendered.html

<nav class='vertical'>
  <h1 class='header vertical--first'>header</h1>
  <ul class='list vertical--last'>
    <li class='item'>
      <a class='link'>blah</a>
    </li>
    <li class='item__nested'>
      <a class='link'>nested!</a>
      <ul class='list'>
        <li class='item'>
          <a class='link'>inside</a>
        </li>
        <li class='item'>
          <a class='link'>inside 2</a>
        </li>
      </ul>
    </li>
    <li class='item__nested'>
      <a class='link'>nested!</a>
      <ul class='list'>
        <li class='item'>
          <a class='link'>inside</a>
        </li>
        <li class='item'>
          <a class='link'>inside 2</a>
        </li>
      </ul>
    </li>
    <li class='item__nested vertical--last'>
      <a class='link'>nested last</a>
      <ul class='list vertical--last'>
        <li class='item vertical--last'>
          <a class='link'>inside</a>
        </li>
      </ul>
    </li>
  </ul>
</nav>