/in/foo.cc: In function 'bool prime(int)':
/in/foo.cc:8:22: error: 'sqrt' was not declared in this scope
  for(int i=int(sqrt(n);i>=2;i--))
                      ^
/in/foo.cc:8:33: error: expected primary-expression before ')' token
  for(int i=int(sqrt(n);i>=2;i--))
                                 ^