code: public Subspace join(Subspace other) {
      long[] newDimensions = joinLastDimensions(other);
      return newDimensions != null ? new Subspace(newDimensions) : null;
    }
docs: Joins this subspace with the specified subspace. The join is only
  successful if both subspaces have the first k-1 dimensions in common (where
  k is the number of dimensions) and the last dimension of this subspace is
  less than the last dimension of the specified subspace.
  
  @param other the subspace to join
  @return the join of this subspace with the specified subspace if the join
  condition is fulfilled, null otherwise.
  @see Subspace#joinLastDimensions(Subspace)