Mapping otherwise conflicting or unmappable keys in terminal Vim
Resources in the network (1, 2, 3) claim that some key
combinations, among which Ctrl-Shift-letter, Ctrl-number,
Ctrl-i/Tab, Ctrl-m/Enter,
Esc/Ctrl-[, cannot be mapped reliably in Vim because the
terminal does not distinguish them from their unmodified counterparts (more
background in this Gilles’ answer and this ASCII table article). As
a concrete example, the maps