*** Input: abbaca
Remove 'bb' => 'aaca'
Remove 'aa' => 'ca'
Output: 'ca'


*** Input: azxxzy
Remove 'xx' => 'azzy'
Remove 'zz' => 'ay'
Output: 'ay'


*** Input: aaaaaaaa
Remove 'aaaaaaaa' => ''
Output: ''


*** Input: aabccba
Remove 'cc' => 'bba'
Remove 'bb' => 'a'
Output: 'a'


*** Input: abcddcba
Remove 'dd' => 'abccba'
Remove 'cc' => 'abba'
Remove 'bb' => 'aa'
Remove 'aa' => ''
Output: ''