static create(openingBracket, child, closingBracket) {
        let length = openingBracket.length;
        if (child) {
            length = lengthAdd(length, child.length);
        }
        if (closingBracket) {
            length = lengthAdd(length, closingBracket.length);
        }
        return new PairAstNode(length, openingBracket, child, closingBracket, child ? child.missingOpeningBracketIds : SmallImmutableSet.getEmpty());
    }
Represents a bracket pair including its child (e.g. `{ ... }`).
Might be unclosed.
Immutable, if all children are immutable.