representing a sub-field or item. The string will follow one of these four formats: 'f:<name>', where <name> is the name of a field in a struct, or key in a map 'v:<value>', where <value> is the exact json formatted value of a list item 'i:<index>', where <index> is position of a item in a list 'k:<keys>', where <keys> is a map of  a list item's key fields to their unique values If a key maps to an empty Fields value, the field that key represents is part of the set.\n\nThe exact format is defined...