allow code fragment to be null
add a field to have all the elements
add a class to analyze modifications on clones
[REFACTORING] rename class
[REFACTORING] rename inner class
include the tasks to detect modifications
add equalizer for diff
[BUG_FIX] add null checking
[BUG_FIX] add null checking
[BUG_FIX] keep the information of LCSs in analyzing the previous version
[BUG_FIX] add null checking
[BUG_FIX] compare Long with equals
[BUG_FIX] compare Long with equals