How to declare a variant with :not() selector?

.component buttonThe element you want to styleThe selector you want to insert:not(.btn)

SASS

+component('#footer')
  +register
    +variant('.mini')
    +element('button')
      +variant(':not(.btn)')

  display: flex

  +____________________________
  +element('button')
    +default
      display: inline-flex
      font-size: 1.25em

    +variant(':not(.btn)')
      +default
        background: orange

      +variant('.mini')
        font-size: 0.75em

CSS

#footer {
  display: flex;
}

#footer button {
  display: inline-flex;
  font-size: 1.25em;
}

#footer button:not(.btn) {
  background: orange;
}

#footer.mini button:not(.btn) {
  font-size: 0.75em;
}

More info: +variant, +default, +element, +register, +component, separator +___