SICP Exercise 4.6 let->combination

Exercise 4.6.  Let expressions are derived expressions, because

(let ((<var1> <exp1>) ... (<varn> <expn>))
  <body>)

is equivalent to

((lambda (<var1> ... <varn>)
   <body>)
 <exp1>
 .
 .
 .
 <expn>)

Implement a syntactic transformation let->combination that reduces evaluating let expressions to evaluating combinations of the type shown above, and add the appropriate clause to eval to handle let expressions.

 SOLUTION

 The code and tests are here.

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