  Due to their immutable log of information, blockchains can be considered as a
transparency-enhancing technology. The immutability, however, also introduces
threats and challenges with respect to privacy laws and illegal content.
Introducing a certain degree of mutability, which enables the possibility to
store and remove information, can therefore increase the opportunities for
blockchains. In this paper, we present a concept for a mutable blockchain
structure. Our approach enables the removal of certain blocks, while
maintaining the blockchain's verifiability property. Since our concept is
agnostic to any consensus algorithms, it can be implemented with permissioned
and permissionless blockchains.
