Ensure that variables used in quos are present in the data and return informative error messages otherwise.
check_quo_vars(xpdb, ..., .source, .problem)
Silent if checks are successful, returns errors otherwise.
An xpose database object.
Name-value pairs of expressions.
The source of the data in the xpdb. Can either be 'data' or an output file extension e.g. 'phi'.
The problem from which the data will be modified