code: func (s Set) Difference(other Set) Set {
  	ret := make(Set)
  	for k := range s {
  		if _, ok := other[k]; !ok {
  			ret[k] = struct{}{}
  		}
  	}
  	return ret
  }
docs: // Difference returns a new Set which is this set with all elements from other
  // removed (i.e. `self - other`).
  //
  // `other` must have the same underlying type as the current set, or this will
  // panic.