Sentence:  The fact that there can not be any limit points of the set except in closed intervals follows from the argument used in Lemma 1 , namely , that near any <WSD>tangent</WSD> point in the C-plane the curves C and **f are analytic , and therefore the difference between them must be a monotone function in some neighborhood on either side of the tangent point . 
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