code: private ImmutableSubstitution<? extends ImmutableTerm> normalizeEta(ImmutableSubstitution<ImmutableTerm> eta,
                                                                          ImmutableSet<Variable> newV) {
          return substitutionTools.prioritizeRenaming(eta, newV);
      }
docs: /*
  Normalizes eta so as to avoid projected variables to be substituted by non-projected variables.
  
  This normalization can be understood as a way to select a MGU (eta) among a set of equivalent MGUs.
  Such a "selection" is done a posteriori.