/in/foo.cc:46:20: error: 'LinkString' has not been declared
  void Index(int k, LinkString& b)
                    ^~~~~~~~~~
/in/foo.cc: In member function 'void SeqString::Index(int, int&)':
/in/foo.cc:59:17: error: request for member 'head' in 'b', which is of non-class type 'int'
    Node* pb = b.head;
                 ^~~~
/in/foo.cc: In function 'void Index(char*, int, char*)':
/in/foo.cc:99:14: error: no matching function for call to 'SeqString::Index(int&, SeqString&)'
  a.Index(k, b);
              ^
/in/foo.cc:46:7: note: candidate: void SeqString::Index(int, int&)
  void Index(int k, LinkString& b)
       ^~~~~
/in/foo.cc:46:7: note:   no known conversion for argument 2 from 'SeqString' to 'int&'