gistfile1.txt

@mixin button-function(param,param,param) {
  // does this
}

.button-default {
  @include button-function();
}
.button-special {
  @include button-function(param:defined);
}                                       

input,button,[role='button'] {
  @extend .button-default;
}                         

.call-to-action {
  @extend .button-special;
}                         

.go-do-this {
  @extend .button-special;
}

.totally-unique-button {
  @include button-function(param,param,param);
}