I am looking at the RequiredFields class and it has this explanation
of what getFields() returns:
* List of fields required from the input. This includes fields that are
* transformed, and thus are no longer the same fields. Using the example 'B
* = foreach A generate $0, $2, $3, udf($1)' would produce the list (0, 0),
* (0, 2), (0, 3), (0, 1). Note that the order is not guaranteed.
The second element of the pair is self-explanatory -- but what is the
first element in the pair?