Contextual higher level classes affecting content of extends in SASS

gistfile1.scss

// In some cases we might want to have the following in SASS:

.some-class-name { 
  width: 50%;
}

.under-a-specific-context {
  .some-class-name {
    width: 35%;
  }
}

// But let's say that you actually have an extendable only extends in SASS:

%some-extendable {
  width: 50%;
}

.some-class-name {
  @extend %some-extendable
}

// You could still do
.under-a-specific-context {
  .some-class-name {
    width: 35%;
  }
}

// but that would only affect that specific use of %some-extendable

// what if you would want to affect all uses of %some-extendable?