example 8a.scss

@mixin fight-with($monster) {
  @if $monster == mechagodzilla {
    .post__title{ &:after { @content } }
  }
  @else if $monster == spacegodzilla {
    @content; 
  }
}

.post--mechagodzilla {
  @include fight-with(mechagodzilla) { content: 'fight with lightning' }
}
.post--spacegodzilla {
  @include fight-with(spacegodzilla) { content: 'fight with Armageddon'; .post_title:after{ content: ":)"} }
}

example 8b.css

.post--mechagodzilla .post__title:after {
  content: "fight with lightning";
}

.post--spacegodzilla {
  content: 'fight with Armageddon';
}
.post--spacegodzilla .post_title:after {
  content: ":)";
}