def get_element_type_indices(t):
    """ returns 3 bool arrays denoting 1D, TRI, QUAD element types
    MPU connectivity array T design is such that:
        1D:  will have -1 in third column
        Tri: will have -1 in fourth column AND won't have -1 in third
        Quad: will have an entry in fourth column
    """
    ids1D = t[:, 2] < 0
    ids2Dtri = (t[:, 3] < 0) & (t[:, 2] >= 0)
    ids2Dquad = (t[:, 3] >= 0)
    return ids1D, ids2Dtri, ids2Dquad