o(1)a(2)…(m) 12 (1)a(2)…o(n)(a(1)o(i2)…a(n)        = (1) (2) ( ) 1 2 n n               =        = (1) (2) ( ) ( ) ( ) ( ) 1 2 1 2 1 2 n n i i i i i i n n           