code: 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;
      }
    }
docs: Returns the logical form for the smallest subtree of the parse which
  completely contains the given span.
  
  @param spanStart
  @param spanEnd
  @return