popChainContext() {
		const context = this.chainContext;

		this.chainContext = context.upper;

		// pop all choice contexts of this.
		for (let i = context.choiceContextCount; i > 0; --i) {
			this.popChoiceContext();
		}
	}
Pop a `ChainExpression` context from the stack. This method is called on
exiting from each `ChainExpression` node. This merges all forks of the
last optional chaining.
@returns {void}