lexical-address-lookup and lexical-address-set! procedures

Write a procedure lexical-address-lookup that implements the new lookup operation. It should take two arguments -- a lexical address and a run-time environment -- and return the value of the variable stored at the specified lexical address. Lexical-address-lookup should signal an error if the value of the variable is the symbol *unassigned* . Also write a procedure lexical-address-set! that implements the operation that changes the value of the variable at a specified lexical address.

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