Write a DTM that computes the function twin:$\{$a$,$b$\}*->\{$a$,$b$\}*$

where twin$(\backslash$lambda $)=\backslash$lambda

twin$($a$0$a$1...$a$($n$2)$a$($n$1)=$ a$0$a$0$a$1$a$1...$a$($n$2)$a$($n$2)$a$($n$1)$a$($n$1)$