// decryptionKey is the parsed and expanded form of a PKE decryption key.
type decryptionKey struct {
	s [k]nttElement // ByteDecode₁₂(dk[:decryptionKeySize])
}

// GenerateKey generates a new decapsulation key, drawing random bytes from
// crypto/rand. The decapsulation key must be kept secret.
func GenerateKey() (*DecapsulationKey, error) {
	// The actual logic is in a separate function to outline this allocation.

  // nested stacks (below or above based on the sign of direction).
  // Returns the largest y coordinate filled.
  function renderStacks(depth, xscale, x, y, places, direction) {
    // Example: suppose we are drawing the following stacks:
    //   a->b->c
    //   a->b->d
    //   a->e->f
    // After rendering a, we will call renderStacks, with places pointing to
    // the preceding stacks.
    //