  The X$^-$ trion is essentially an electron bound to an exciton. However, due
to the composite nature of the exciton, there is no way to write an
exciton-electron interaction potential. We can overcome this difficulty by
using a commutation technique similar to the one we introduced for excitons
interacting with excitons, which allows to take exactly into account the
close-to-boson character of the excitons. From it, we can obtain the X$^-$
trion creation operator in terms of exciton and electron. We can also derive
the X$^-$ trion ladder diagram between an exciton and an electron. These are
the basic tools for future works on many-body effects involving trions.
