// '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.
//

  //   - 'set': `X + Y` performs a union where the array positions of all elements in `X` are preserved and
  //     non-intersecting elements in `Y` are appended, retaining their partial order.
  //   - 'map': `X + Y` performs a merge where the array positions of all keys in `X` are preserved but the values