This rule applies whenever an `@if` directive is followed by one or more `@else if` directives; the final `@else if` directive should be followed by an `@else` directive.

The requirement for a final `@else` directive is defensive programming.

The `@else` directive should either take appropriate action or contain a suitable comment as to why no action is taken.

## Noncompliant Code Example

    @if $x == 0 {
      do-something();
    } @else if $x == 1 {
      do-something-else();
    }

## Compliant Solution

    @if $x == 0 {
      do-something();
    } @else if $x == 1 {
      do-something-else();
    } @else {
      @error "Unexpected value for $x: #{$x}";
    }