This package provides an implementation of mutable Fenwick trees. It is maximally generic. Each operation of Fenwick tree is implemented using a subset of constraints from Semigroup, Monoid, or Commutative, chosen based on the nature of each operation. It is fast and efficient. With ArrayC and VectorC from this package, it is possible to use unboxed arrays and vectors for newtypes that implement a custom algebra (e.g. Sum, Product or Xor). An implementation using this library can be as fast as a C/C++ implementation.