Please write clearly. Thanks!

Consider the language LETREC. In LETREC we are able to write recursive procedures such as letrec double(x) = if zero?(x) then 0 else -((double -(x,l)), -2) in (double 6) Extend LETREC to allow the declaration of any number of mutually recursive unary procedures, for example: letrec even(x) = if zero?(x) then 1 else (odd -(x,l)) odd(x) = if zero?(x) then 0 else (even -(x,l)) in (odd 13) Write the solution to this exercise using DrRacket. Start by implementing the following modification. In the file lang.scm of LETREC, replace the specification for > 9 letrec * * (expression ("letrec" identifier "(" identifier ")" "=" expression "in" expression) letrec-exp) with the following code: (expression ("letrec" (arbno identifier "(" identifier ")" "=" expression) "in" expression) letrec-exp) Consider the language LETREC. In LETREC we are able to write recursive procedures such as letrec double(x) = if zero?(x) then 0 else -((double -(x,l)), -2) in (double 6) Extend LETREC to allow the declaration of any number of mutually recursive unary procedures, for example: letrec even(x) = if zero?(x) then 1 else (odd -(x,l)) odd(x) = if zero?(x) then 0 else (even -(x,l)) in (odd 13) Write the solution to this exercise using DrRacket. Start by implementing the following modification. In the file lang.scm of LETREC, replace the specification for > 9 letrec * * (expression ("letrec" identifier "(" identifier ")" "=" expression "in" expression) letrec-exp) with the following code: (expression ("letrec" (arbno identifier "(" identifier ")" "=" expression) "in" expression) letrec-exp)