  A detailed review of the $p,q$-duality for Calogero system and its
generalizations is given. For the first time, we present some of
elliptic-trigonometric Hamiltonians dual to the elliptic Ruijsenaars
Hamiltonians (i.e. trigonometric-elliptic ones), and explain their relations to
the bi-elliptic Koroteev-Shakirov (KS) model. The most interesting self-dual
double-elliptic (DELL) system remains a mystery, but we provide a clearer
formulation of the problem and describe the steps that are still to be done.
