code: function LiteralBunch_operator(z_literal_filter) {
  	// user wants to filter literal by language or datatype
  	if('string' === typeof z_literal_filter) {
  		// by language tag
  		if('@' === z_literal_filter[0]) {
  			// ref language tag
  			let s_language = z_literal_filter.slice(1).toLowerCase();
  
  			// apply filter
  			this.terms = this.terms.filter((h_term) => {
  				return s_language === h_term.language;
  			});
  		}
  		// by datatype
  		else {
  			// resolve datatype iri
  			let p_datatype = this.graph.resolve(z_literal_filter);
  
  			// apply filter
  			this.terms = this.terms.filter((h_term) => {
  				return p_datatype === h_term.datatype;
  			});
  		}
  	}
  	// user wants to filter literal by a function
  	else if('function' === typeof z_literal_filter) {
  		// apply filter
  		this.terms = this.terms.filter(z_literal_filter);
  	}
  
  	// results / no filtering
  	return this;
  }
docs: mutates the LiteralBunch exactly once with the given filter argument