The transformation function for this command.
a new transformed data structure. May be
Throwable if any exceptional case occurs
Immutability of the data structure cannot be enforced. Therefore ensuring immutability (or at least apparent immutability) lies in the responsibility of the user.
Chains this command with second one.
the output type of the second command
the command to chain with this one. May not be
a MutationCommand with the functionality of this one