code: func removeChild(parent Context, child canceler) {
  	p, ok := parentCancelCtx(parent)
  	if !ok {
  		return
  	}
  	p.mu.Lock()
  	if p.children != nil {
  		delete(p.children, child)
  	}
  	p.mu.Unlock()
  }
docs: // removeChild removes a context from its parent.