def sum_lists(arg1, arg2):
    """
    Return a list that is the element-wise sum of the elements of the
    two lists of equal size passed as arguments.

    Lists can be used to represent vectors of numbers as in Linear
    Algebra. However, as you can test in a python shell, for example,
    `ipython`, the effect the operator `+` is not really the expected
    one in linear algebra.

    Parameters
    ----------
    arg1 : list(numeric)
        a list of numbers
    arg2 : list(numeric)
        a list of numbers

    Returns
    -------
    list(int)
        a list containing the element-wise sum of the two lists

    Raises
    ------
    ValueError
        If the two lists are not of equal size
        that are relevant to the interface.


    Examples
    --------
    
    >>> va = [1, 1, 1]
    >>> vb = [2, 2, 2]
    >>> sum_lists(va,vb)
    [3, 3, 3]
    """
    if (len(arg1) != len(arg2)):
        raise ValueError("List arguents of different lenghts")
    else:
        return [arg1[i] + arg2[i] for i in range(len(arg1))]