Fold

• hadDefinition https://en.wikipedia.org/wiki/Fold_(higher-order_function)
  • the order in which the elements are combined may influence the final result's value. On lists, there are two obvious ways to carry this out: either by combining the first element with the result of recursively combining the rest (called a right fold), or by combining the result of recursively combining all elements but the last one, with the last element (called a left fold).