def sort_terms(terms):
    """
    Returns a list of two sympy expressions where the expression is
    positive and the second expression is negative.

    Parameters
    ----------
    terms : list of sympy expressions
        A list with length of 2 where one element is positive and
        the other is negative (starts with a minus symbol)

    Returns
    -------
    tuple of sympy expressions
        A tuple where the first element is positive and the
        second is negative.
    """

    neg = None
    pos = None
    for term in terms:
        if str(term)[0] == '-':  # negative terms should start with a '-'
            neg = term
        else:
            pos = term
    assert neg, 'No negative terms ' + str(terms)
    assert pos, 'No positive terms ' + str(terms)
    return pos, neg