#NEWLINE# def alias_diff( refcounts_before , refcounts_after ) : #NEWLINE# #INDENT# '\n    Given the before and after copies of refcounts works out which aliases\n    have been added to the after copy.\n    ' #NEWLINE# return set ( ( t for t in refcounts_after if ( refcounts_after [ t ] > refcounts_before . get ( t , ( - 1 ) ) ) ) )