Generates a list categorizing items based on their marks: items to be added ('+'), items to be dropped ('-'), items to be used (unmarked or marked with '+'), and indices of '+' items within the used category.
process_vector(vector)
A list with the following components:
Items marked with '+'
Items marked with '-'
Items that are unmarked or marked with '+'
TRUE for items marked with '+' within the $use category
[mandatory] (character) A character vector containing elements marked with '+' and '-' prefixes.