“…Recently, inspired by the squared eigenfunction symmetries [23,24], the authors proposed a systematic approach to construct new (2 + 1)-dimensional integrable systems, by extending certain flow of the system with the help of squared eigenfunction symmetries. In this way, the authors have found many new integrable hierarchies, which are called the 'extended integrable hierarchies', such as the extended KP and mKP, BKP, CKP hierarchies, the extended discrete KP hierarchy, the extended q-deformed KP hierarchy as well as the extended two-dimensional Toda lattice hierarchy [15,17,19,20,[33][34][35].…”