code: private boolean isAbsent(QuotedID attribute) {
          ImmutableSet<RelationID> occurrences = attributeOccurrences.get(attribute);
          return (occurrences == null) || occurrences.isEmpty();
      }
docs: checks if the attributeOccurrences contains the attribute
  
  @param attribute a  {@link QuotedID}
  @return true if attributeOccurrences contains the attribute otherwise false