code: public Feature newProperty(String lemma, List<Span<Term>> references) {
  	String newId = idManager.getNextId(AnnotationType.PROPERTY);
  	Feature newProperty = new Feature(newId, lemma, references);
  	annotationContainer.add(newProperty, Layer.PROPERTIES, AnnotationType.PROPERTY);
  	return newProperty;
      }
docs: Creates a new property. It assigns an appropriate ID to it. The property is added to the document.
  @param lemma the lemma of the property.
  @param references different mentions (list of targets) to the same property.
  @return a new coreference.