Preface; Note to the reader; Terminology, notations and conventions used; List of special notation; 0. Preliminaries on modules; 1. Principal ideal domains; 2. Firs, semifirs and the weak algorithm; 3. Factorization; 4. 2-firs with a distributive factor lattice; 5. Modules over firs and semifirs; 6. Centralizers and subalgebras; 7. Skew fields of fractions; Appendix; Bibliography and author index; Subject index.