Advanced XForm Recipes

#20 and #222 pull in some form design documentation that talks about raw XML. In general, this is not used much. However, there are some real cases in which it might be helpful.

Let's use this issue thread to collect those and to eventually add them to an existing document or create a new one.

Perform a complex calculation and show it to the user without introducing a new `calculate` field: https://forum.opendatakit.org/t/is-output-value-an-instance-node-or-xpath/10262/6

Iterate over a list of choices identified by a first question in a cascading select: https://forum.opendatakit.org/t/repeat-questions-depending-upon-selection/10033/8

Build a dynamic list of select choices based on previous form values: https://forum.opendatakit.org/t/generate-multiple-choice-alternatives-based-on-the-answer-of-a-previous-question/10269/4?u=ln

Start by reading the existing form design documentation referenced by #20 and #222, then review the three linked forum discussions for concrete raw XML use cases. Done means agreeing on the useful recipes and adding them to an existing document or a new one.

Written by the indexing model from the issue text.