def intersect_intervals(a, b):
    i = j = 0
    output = []
    while i < len(a) and j < len(b):
        start = max(a[i][0], b[j][0])
        end = min(a[i][1], b[j][1])
        if start < end:
            output.append([start, end])
        if a[i][1] <= b[j][1]:
            i += 1
        else:
            j += 1
    return output