code: public StrBuilder append(final StrBuilder str) {
          if (str == null) {
              return appendNull();
          }
          final int strLen = str.length();
          if (strLen > 0) {
              final int len = length();
              ensureCapacity(len + strLen);
              System.arraycopy(str.buffer, 0, buffer, len, strLen);
              size += strLen;
          }
          return this;
      }
docs: Appends another string builder to this string builder.
  Appending null will call {@link #appendNull()}.
  
  @param str  the string builder to append
  @return this, to enable chaining