and AGNs (nuclear X-ray power vs. SFR or stellar mass), redshift evolution of the cosmic SFR and BH mass density. However, the picture requires further validation especially in the early stages of the coevolution, that plainly are not easy to pinpoint given the sensitivity and resolution limits of current X-ray and far-IR facilities.

Fortunately, strong gravitational lensing by foreground objects offers an extraordinary potential to advance our understanding of such elusive early stages. It not only yields flux boosting that can reach factors $\mu\ge 10$, allowing us to explore regions of the luminosity/redshift space that would otherwise be unaccessible, but also stretches the images by factors $\simeq \mu^{1/2}$, allowing the study of fine spatial details.

In fact, observations have shown that almost all high-redshift ($z>1$) dust-obscured star forming galaxies selected in the sub-millimetre (submm galaxies) with flux density above $\sim 100$ mJy at 500 $\mu$m are gravitationally lensed by a foreground galaxy or a group/cluster of galaxies (see Negrello et al. 2010, 2014, 2017; Wardlow et al. 2013; Nayyeri et al. 2016). These submm bright sources are rare ($<0.3$ deg$^{-2}$ at $S_{500\mu m}>100$ mJy, Negrello et al. 2007) and therefore only detectable in wide-area submm surveys, like those by the 