Consider the problem of maximizing u(c,l) subject to pc+wl=wT+Y, where c is consumption, l is leisure time, T is the total time endowment, and Y is non-wage income. Prove that the labour supply function is downward-sloping only if leisure is a normal good.

还是一道英文微观经济题，真心请教各位前辈，如能给出详细解答过程更好，若嫌麻烦，能给出解答步骤也行

谢谢大家~