有二个函数不明白，求人讲解

namespace {
    void dump_cdr_internal(const Sexp::Cell *cell,
                   std::ostream *os);

    void dump_internal(const Sexp::Cell *cell,
               std::ostream *os) {
      if (!cell) {
          *os << "NIL";
      } else {
      if (cell->is_cons()) {
         *os << '(';
           dump_internal(cell->car(), os);
           dump_cdr_internal(cell->cdr(), os);
         *os << ')';
     } else if (cell->is_atom()) {
         *os << cell->atom();
      }
    }
  }

void dump_cdr_internal(const Sexp::Cell *cell,
                   std::ostream *os) {
    if (!cell) {
           return;
    }
    if (cell->is_cons()) {
       *os << ' ';
         dump_internal(cell->car(), os);
         dump_cdr_internal(cell->cdr(), os);
   } else {
      *os << ' ';
         dump_internal(cell, os);
   }
  }
}

const Sexp::Cell *Sexp::read(char **begin, const char *end) {
     comment(begin, end);
     if (*begin >= end) {
        return 0;
     }
    const int r = next_token(begin, end, '(');
       if (r == 1) {
           return read_car(begin, end);
    } else if (r == 0) {
          return read_atom(begin, end);
    }
      return 0;
 }

void Sexp::dump(const Sexp::Cell *cell, std::ostream *os) {
     dump_internal(cell, os);
     *os << std::endl;
}

void Sexp::free() {
     cell_freelist_.free();
     char_freelist_.free();
}

int Sexp::next_token(char **begin, const char *end, const char n) {
   char c = 0;
   do {
      c = **begin;
      ++(*begin);
  } while (isspace(c));

  if (*begin >= end) {
        return -1;
  }

  if (c == n) {
       return 1;
  } else {
      --(*begin);
      return 0;
  }
}

void Sexp::comment(char **begin, const char *end) {
    const int r = next_token(begin, end, ';');
    if (r == 1) {
      while (*begin < end) {
       const char c = **begin;
        ++(*begin);
       if (c == '\r' || c == '\n') {
            break;
        }
     }
       return comment(begin, end);
   }
}

const Sexp::Cell *Sexp::read_cdr(char **begin, const char *end) {
    comment(begin, end);
    const int r = next_token(begin, end, ')');
    if (r == 1) {
          return 0;
   } else if (r == 0) {
        return read_car(begin, end);
   }
         return 0;
}

 const Sexp::Cell *Sexp::read_car(char **begin, const char *end) {
    comment(begin, end);
    const int r = next_token(begin, end, ')');
    if (r == 1) {
         return 0;
    } else if (r == 0) {
         Cell *cell = cell_freelist_.alloc(1);
         cell->set_car(this->read(begin, end));
         cell->set_cdr(read_cdr(begin, end));
          return cell;
    }
         return 0;
}

const Sexp::Cell *Sexp::read_atom(char **begin, const char *end) {
     comment(begin, end);
     char c = **begin;
     const char *p = *begin;
      ++(*begin);
      if (isspace(c) || c == '(' || c == ')') {
            return 0;
     } else if (*begin >= end) {
          return 0;
     } else {
         while (true) {
          c = **begin;
          ++(*begin);
          if (isspace(c) || c == '(' || c == ')' || *begin >= end) {
              --(*begin);
              const size_t length = static_cast<size_t>(*begin - p);
              char *tmp = char_freelist_.alloc(length + 1);
              std::copy(p, p + length, tmp);
              tmp[length] = '\0';
              Cell *cell = cell_freelist_.alloc(1);
              cell->set_atom(tmp);
              return cell;
          }
       }
   }
         return 0;
}