def local_indicies(lead_step, child_steps):
    """
    Returns a tuple of indices surrounding the lead_step
    """
    imin = 0
    imax = len(child_steps) - 1
    mid = 0
    while imin < imax:
        mid = (imax + imin)/2
        if(child_steps[mid][0] < lead_step):
            imin = mid + 1
        else:
            imax = mid
    if child_steps[mid][0] < lead_step:
        return (mid, mid+1)
    return mid-1,mid