code: protected void appendColumns (Iterable<String> columns, StringBuilder buf) {
          int ii = 0;
          for (String column : columns) {
              if (ii++ > 0) {
                  buf.append(", ");
              }
              buf.append(columnSQL(column));
          }
      }
docs: Escapes {@code columns} with {@link #columnSQL}, appends (comma-sepped) to {@code buf}.