presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitUnboundedFrame
class AstBuilder:    @Override
    public Node visitUnboundedFrame(SqlBaseParser.UnboundedFrameContext context)
    {
        return new FrameBound(getLocation(context), getUnboundedFrameBoundType(context.boundType));
    }