if (this.expandOnce(true) === false) {  // fully expanded
                const token = this.stack.pop();
                if (token.treatAsRelax) {
                    // the expansion of \noexpand is the token itself
                    token.noexpand = false;
                    token.treatAsRelax = false;
                }
                output.push(token);
            }
Fully expand the given token stream and return the resulting list of
tokens.  Note that the input tokens are in reverse order, but the
output tokens are in forward order.