import math


def angle_to(tup1,tup2):
    """The angle to tup2 from tup1, measured against 'straight up' clockwise, in radians.

    Tuples are given in a coordinate system with (0,0) top left, (1,0) is one step right, (0,1) is one step down,
    and (1,1) is down right.

    >>> angle_to((1,1),(1,0))/math.pi
    0.0
    >>> angle_to((1,1),(2,0))/math.pi
    0.25
    >>> angle_to((1,1),(1,2))/math.pi
    1.0
    >>> angle_to((1,1),(-1,-1))/math.pi
    1.75
    """
    x = tup2[0] - tup1[0]
    y = tup1[1] - tup2[1]
    angle_to_positive_x_axis = math.atan2(y,x)
    angle_to_straight_up = (math.pi/2) -angle_to_positive_x_axis
    positive_angle_to_straight_up = angle_to_straight_up % (2*math.pi)
    return positive_angle_to_straight_up