def _eig_leftvec_add_mps(lv, lt1, lt2):
    """Add one column to the left vector (MPS version)"""
    # MPS 1: Interpreted as |psiXpsi| part of the operator
    # MPS 2: The current eigvectector candidate
    # NB: It would be more efficient to store lt1.conj() instead of lt1.