* ```#acc``` represents a value that you want to include in your single output when you reduce an array. * ```#index``` represents the current array index, along with array index numbers ```#index2, ...

Solves a problem with conversion of retrieved from a database sets of data to a nested data structure (i.e. navigation tree).

For example, imagine that a score table in a game needs to record ten scores. One way to do this is to have a separate variable for each score: score_0 score_1 score_2 score_3 score_4 score_5 score_6 ...