def make_diagonal(core, entry):
    """
    Outputs a list of 3 elements to act as the diagonal of an identity matrix.
    the first and two elements are equal and will serve as the row and column
    indexes, while the third element is a dash.
    Called by: get_knots()
    """
    line = []
    line.append(core[entry].get_parent().id[1])
    line.append(core[entry].get_parent().id[1])
    line.append('-')
    return line
