What I was trying to do was to use (clumsily, I understand from your, @stevengj and @StefanKarpinski’s answers) an idea seen in an ‘other post’ of which I don’t remember the link in which the hash() and ==() functions were redefined for a new User Type.
The aim was to use a “special dictionary” to solve this problem.
The idea was to emulate the groupby function (to create equivalence classes and put them together as requested by the OP (at least as I understood it), via the mergewith!(combine, d1,d2, … .)).