This a list of some interesting tests your program could 
have handle:

a. basic transactions (insertion of nonexisting identifier, 
   deletion of existing identifier, and modification of 
   existing identifier);

b. multiple reference to the same identifier in different
   transaction files or in the same transaction file;

c. erroneous transactions (insertion of existing identifier,
   deletion of nonexisting identifier, and modification of
   nonexisting identifier);

d. sintax errors, type transaction out of range, etc.