docs: protected CLIQUEUnit join(CLIQUEUnit other, double all, double tau) {
      if(other.dimensionality() != this.dimensionality()) {
        return null;
      }
      // n-1 dimensions must be the same:
      int e = dims.length - 1;
      if(!checkDimensions(other, e)) {
        return null;
      }
      if(dims[e] >= other.dims[e]) {
        return null;
      }
  
      HashSetModifiableDBIDs resultIDs = DBIDUtil.newHashSet(this.ids);
      resultIDs.retainAll(other.ids);
  
      if(resultIDs.size() / all < tau) {
        return null;
      }
      return new CLIQUEUnit(this, other.dims[e], other.bounds[e << 1], other.bounds[(e << 1) + 1], resultIDs);
    }