def find_anchors_with_starting_structure(model):
    """
    Search the model for anchors which have a defined starting 
    structure.
    
    Parameters
    ----------
    model : Model()
        A Seekr2 Model object with insufficiently filled anchor
        starting structures.
        
    Returns
    -------
    anchors_with_starting_structures : str
        A list of integers representing anchor indices where a
        starting structure was found.
    """
    anchors_with_starting_structures = []
    for i, anchor in enumerate(model.anchors):
        assert i == anchor.index
        if anchor.amber_params is not None:
            if anchor.amber_params.pdb_coordinates_filename:
                anchors_with_starting_structures.append(anchor.index)
        
        elif anchor.forcefield_params is not None:
            anchors_with_starting_structures.append(anchor.index)
        
        elif anchor.charmm_params is not None:
            raise Exception("Charmm systems not yet implemented")
        
    
    assert len(anchors_with_starting_structures) > 0, "No anchors with "\
        "starting structures were found in this model."
    return anchors_with_starting_structures