Sentence:  The fact that there can not be any limit points of the <WSD>set</WSD> except in closed intervals follows from the argument used in Lemma 1 , namely , that near any tangent 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:  (mathematics) an abstract collection of numbers or symbols