( W-bound C = W-bound (Lower_Arc C) & E-bound C = E-bound (Lower_Arc C) ) by Th19, Th20;
hence Lower_Arc C meets Vertical_Line (((W-bound (Lower_Arc C)) + (E-bound (Lower_Arc C))) / 2) by JORDAN6:62; :: according to JORDAN21:def 1 :: thesis: verum