encode procedure takes as arguments a message and a tree and
produces the list of bits that gives the encoded message.
(define (encode message tree) (if (null? message) '() (append (encode-symbol (car message) tree) (encode (cdr message) tree))))
Encode-symbol is a procedure, which you must write, that returns the list of bits that encodes a given symbol according to a given
tree. You should design
encode-symbol so that it signals an
error if the symbol is not in the tree at all. Test your procedure by
encoding the result you obtained in exercise 2.67 with the sample tree and seeing whether it is the same as the original sample message.