Congrats @clauderoux! -- I've been working on similar fusions on and off during the years.

So "?X" is not true logical variable, but more of a part of a pattern expression? -- in other words, a logical var is a first-class object that can be stored in a data-structure (possibly in an unbound state) and be bound at a later time, and perhaps unbound and re-bound again if there is backtracking. So in Tamgu, "parent(?X,?Y)" acts as a one-shot "find" if the LHS is atomic, and acts as a generator (similar to a possibly nested list comprehension) if the LHS is a collection (but no inter-statement backtracking points are established), correct? So is the generation of the sequence eager/realized or lazy/by-need?