For the most part, this is a well written paper. (The only real flaw is the title “Four concepts become one”. The authors were so anxious to pronounce this idea that they used a single subject “four concepts”. The paper is about that - demonstrating the link among separate concepts of rank of a nonlinear system used to determine right and left invertibility and dynamic decoupling. The paper is of interest, but gives no examples and is dry. No attempt is made to give insight into or physical interpretation of the various rank concepts beyond the definitions.