def subsequent_pairs(l):
    """
    Return subsequent pairs of values in a list <l>, i.e. [(x1, x2), (x2, x3), (x3, x4), .. (xn-1, xn)] for a
    list [x1 .. xn]
    """
    
    return [(l[i-1], v) for i, v in enumerate(l) if i > 0]