fun fact 0 = 1  | fact n = n * fact(n-1); fact 4; fun append2 ([], ys) = ys  | append2 (x::xs, ys) = x :: append2 (xs,ys); append2( [1,2,3], [4,5,6] ); fun append [] ys = ys  | append (x :: xs) ys = x :: append xs ys; append [1,2,3] [4,5,6]; fun reverse xs =  let fun loop [] ys = ys        | loop (x::xs) ys = loop xs (x::ys)  in loop xs [] end; reverse [1,2,3,4];reverse [true,false]; val op @ = append2;infixr 5 @;[1,2,3] @ [4,5,6];