function disallow_unparenthesized_sequences(expression, source) {
	if (expression.type === 'SequenceExpression') {
		let i = /** @type {number} */ (expression.start);
		while (--i > 0) {
			const char = source[i];
			if (char === '(') break; // parenthesized sequence expressions are ok
			if (char === '{') e.attribute_invalid_sequence_expression(expression);
		}
	}
}
@param {Expression} expression
@param {string} source