Returns the angle whose tangent is the specified number.
 Parameters

y  The y coordinate of a point. 
x  The x coordinate of a point. 
 Returns
 An angle, A, measured in radians, such that PI <= A <= PI, and tan(?) = y / x, where (x, y) is a point in the Cartesian plane.
Observe the following:
 For (x, y) in quadrant 1, 0 < A < PI/2.
 For (x, y) in quadrant 2, PI/2 < A <= PI.
 For (x, y) in quadrant 3, PI < A < PI/2.
 For (x, y) in quadrant 4, PI/2 < A < 0.
 For points on the boundaries of the quadrants, the return value is the following:
 If x is 0 and y is not negative, A = 0.
 If x is 0 and y is negative, A = PI.
 If x is positive and y is 0, A = PI/2.
 If x is negative and y is 0, A = PI/2.