/in/foo.cc: In function 'BigInteger operator+(BigInteger&, BigInteger&)':
/in/foo.cc:31:39: warning: unused variable 'len' [-Wunused-variable]
   int lena=a.s.size(),lenb=b.s.size(),len=min(lena,lenb);
                                       ^~~
/in/foo.cc: In function 'BigInteger operator-(const BigInteger&, const BigInteger&)':
/in/foo.cc:48:2: warning: no return statement in function returning non-void [-Wreturn-type]
  }
  ^
/in/foo.cc: In function 'BigInteger operator*(const BigInteger&, const BigInteger&)':
/in/foo.cc:50:2: warning: no return statement in function returning non-void [-Wreturn-type]
  }
  ^
/in/foo.cc: In function 'BigInteger operator/(const BigInteger&, const BigInteger&)':
/in/foo.cc:52:2: warning: no return statement in function returning non-void [-Wreturn-type]
  }
  ^
/in/foo.cc: In function 'std::ostream& operator<<(std::ostream&, const BigInteger&)':
/in/foo.cc:2:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 #define rep(i,l,r) for(int i=(l);i<=(r);i++)
                                   ^
/in/foo.cc:79:3: note: in expansion of macro 'rep'
   rep(j,0,strlen(buf)-1) out<<buf[j];
   ^~~