def parens(expr):
    """
    >>> from nl.arith import parens
    >>> parens('uno')
    'uno'
    >>> parens('(uno (dos tres) cuatro)')
    ['uno', '(dos tres)', 'cuatro']
    >>> parens('(uno (dos tres) (ho ho (he (ha ha))) cuatro)')
    ['uno', '(dos tres)', '(ho ho (he (ha ha)))', 'cuatro']
    """
    if expr[0] != '(':
        return expr
    depth = 0
    term = ''
    terms = []
    for c in expr:
        if depth == 1 and c == ' ':
            terms.append(term)
            term = ''
        elif c == '(':
            depth += 1
            if depth > 1:
                term += c
        elif c == ')':
            depth -= 1
            if depth > 0:
                term += c
        else:
            term += c
    terms.append(term)
    if depth != 0:
        raise ValueError('wrong arithmetic expression')
    return terms
