There is a deer in the garden, munching the fallen Bramleys. He moved deliberately, like a dancer. Seeing me, he treated me to a full frontal stare, the only movement an occasional licking of the lips. Judging me to be relatively harmless, he returned to hoovering up the apples. I approached slowly when his head was down. He stopped and looked again, and then (this is what I was waiting for) made a swift exit up the slope and over the fence, his movements an expression of easily contained power, with magical, Nureyev style slowness in mid leap.
Refreshed, I came back in and sat down in front to the screen to learn more about derivatives.