As the bell signals the end of another tedious and sultry day, there is hurried gabbling, shuffling of papers, textbooks squashed with little dignity into bulky schoolbags, a snatch or two of nervous laughter, leather shoes thundering across the carpet, and the door closing with its plaintive squeal. Silence. She remains at the desk, in the right-hand corner of the classroom. A bead of sweat trickles down the side of her cheek, but she brushes it off before it lands on the grid paper. She scratches the paper with her pencil; solid, consistent strokes of a well-practiced swimmer approaching the final lap. She finds the last coordinate, plots the last point. As expected, the points join in smooth curves. The hyperbola and parabola intersect perfectly. Satisfied, she takes her time …