Sentence:  In the neighborhood of an end point of an interval of <WSD>tangent</WSD> points in the f-plane the function is two valued or no valued on one side , and is a single valued function consisting entirely of tangent points on the other side . 
Meaning of the word marked with <WSD> tags:  a straight line or plane that touches a curve or curved surface at a point but does not intersect it at that point