The rules can employ a scoring system that is based on molecular structural features, which can either facilitate the reaction (e.g., increase the predicted yield), or impede it. The syntactic elements that increase the transform's baseline score are the so-called ADD statements, and the SUBTRACT statements as their obvious counterpart. A third, related, syntactic element that is available if the author of a rule deems that structural features would make the reaction entirely unlikely to succeed is the KILL statement, whose meaning and effect is obvious. ADD and SUBTRACT values have traditionally been assigned in increments of five, and typically range from 5 to 30. In spite of their quantitative appearance, they are essentially qualitative human assessments.