create an abstract class which is the base class of all the elements to be registered into the db
create a class for a revision
[minor change] add a comment
create a class for a source file
create a class for a code fragment
[refactoring
] change the names of fields and getters
create a class for a clone set
create an abstract class to represent a link between two elements
create a class for links of code fragments
create a class for links of code fragments
create a class for a link between two clone sets
create a class for a genealogy of clones