Improve shape structures
As explained by @paskevyc in #276 (closed), the following could improve the situation with respect to the part of the shapes that is really needed for pairing:
"The idea is that a transformation that produces several tasks that only differ in premises (e.g., destruct or split_vc when the goal is no more splittable) should mark these differing premises, and only them, to be included into the shape."