def distance_between(a, b):
    """ A function to calculate the distance between agent a and agent b.
    Args:
        a: A list of two coordinates for orthoganol axes.
        b: A list of two coordinates for the same orthoganol axes as a.
    Returns:
        The straight line distance between the a and b in the an plane given 
        by two orthoganol axes.
    """
    distance = ((a[1] - b[1])**2 + (a[0] - b[0])**2)**0.5
    ##print("distance =", str(distance))
    return distance