add procedures element-of-set?, adjoin-set, union-set, and intersection-set for ordered set
add an implementation of adjoin-set using the ordered represetation
add two procedures for converting a binary tree to a list
add the anlysis of the procedure list->tree
add an empty file
add the implementation of the procedure lookup
add the result of sample message
add the procedure encode-symbol
add the procedure generate-huffman-tree
add the result of encoded message
add the procedures of huffman tree
add the procedures of tree set