docs: public SpannedExpression getLogicalFormForSpan(int spanStart, int spanEnd) {
      CcgParse spanningParse = getParseForSpan(spanStart, spanEnd);
      Expression2 lf = spanningParse.getPreUnaryLogicalForm();
      if (lf != null) {
        return new SpannedExpression(spanningParse.getHeadedSyntacticCategory(),
            spanningParse.getLogicalForm(), spanningParse.getSpanStart(), spanningParse.getSpanEnd());
      } else {
        return null;
      }
    }