how canceling a cancelation could arise organically

cancel-cancel.js

for (let x of foo()) {
  ...
  // generator gets canceled at some point before it completes the iteration
  break;
  ...
}

function* foo() {
  ...
  try {
    // imagine cancellation happens here while delegating
    ... yield* delegate() ...
  } catch (e) {
    // catch errors and keep executing
    log(e)
  }
  // yield more values after the delegation
  ... yield ...
}

// delegate generator function has some internal cleanup code
function* delegate() {
  ...
  try {
    // imagine cancellation happens here inside the try
    ... yield ...
  } finally {
    // cleanup code uses a helper
    ... helper() ...
  }
  ...
}

// some helper function that can sometimes throw an exception
function helper() {
  ... throw ...
}