  Today, the brightest cluster galaxies (BCGs) are passive and very massive
galaxies at the center of their clusters, and they still accrete mass through
swallowing companions and gas from cooling flows. However their formation
history is not well known. We report CO(4$\rightarrow$3) and continuum map
observations of the SpARCS1049+56 BCG at $z=1.709$, one of the most distant
known BCGs. Our observations yield $M_{{\rm H}_2}<1.1\times10^{10}M_\odot$ for
the BCG; while in CO(4$\rightarrow$3), we detect two gas-rich companions at the
northeast and southeast of the BCG, within 20 kpc, with ${L^\prime_{\rm
CO(4\rightarrow3)}=(5.8\pm0.6)\times10^{9}}$ K km s$^{-1}$ pc$^2$ and
$(7.4\pm0.7)\times10^{9}$ K km s$^{-1}$ pc$^2$, respectively. The northern
companion is associated with a pair of merging cluster galaxies, while the
southern one shows a southern tail in CO(4$\rightarrow$3), which was also
detected in continuum, and we suggest it to be the most distant jellyfish
galaxy for which ram pressure stripping is effectively able to strip off its
dense molecular gas. This study probes the presence of rare gas-rich systems in
the very central region of a distant cluster core, which will potentially merge
into the BCG itself. Currently, we may thus be seeing the reversal of the star
formation versus density relation at play in the distant universe. This is the
first time the assembly of high-$z$ progenitors of our local BCGs can be
studied in such great detail.
