text: class PGroup {
  	constructor(array) {
  		this.members = array;
  	}
  
  	add(element) {
  		if (this.has(element)) return this;
  		return new PGroup(this.members.concat([element]));
  	}
  
  	delete(element) {
  		if (!this.has(element)) return this;
  		return new PGroup(this.members.filter(e => e !== element));
  	}
  
  	has(element) {
  		return this.members.includes(element);
  	}
  }
  
  PGroup.empty = new PGroup([]);
  