def _distance(coord1, coord2):
    """
    Return the distance between two points, `coord1` and `coord2`. These
    parameters are assumed to be (x, y) tuples.
    """
    xdist = coord1[0] - coord2[0]
    ydist = coord1[1] - coord2[1]
    return sqrt(xdist*xdist + ydist*ydist)