[minor change] reformat the print format
create a class for commits
[refactoring] move package
add fields to commit for identifiers
make all functions use commit
[bug fix]
create a class to detect chains of elements
create an abstract class to convert chains to genealogies
add a field to represent revisions of chains
create a function to create new instances of code fragment genealogies
create a function to create fragment genealogies from the given chains