foo.cc: In function 'std::istream& operator>>(std::istream&, Matrix&)':
foo.cc:11:22: error: 'i' was not declared in this scope; did you mean 'in'?
   11 |                 _for(i,0,mat.dim_x)
      |                      ^
      |                      in
foo.cc:11:17: error: '_for' was not declared in this scope
   11 |                 _for(i,0,mat.dim_x)
      |                 ^~~~
foo.cc: In member function 'Matrix Matrix::operator*(const Matrix&) const':
foo.cc:29:30: error: 'i' was not declared in this scope
   29 |                         _for(i,0,dim_x)
      |                              ^
foo.cc:29:25: error: '_for' was not declared in this scope
   29 |                         _for(i,0,dim_x)
      |                         ^~~~
foo.cc: In member function 'Matrix Matrix::transpose()':
foo.cc:43:22: error: 'i' was not declared in this scope
   43 |                 _for(i,0,dim_x)
      |                      ^
foo.cc:43:17: error: '_for' was not declared in this scope
   43 |                 _for(i,0,dim_x)
      |                 ^~~~
foo.cc: In function 'std::ostream& operator<<(std::ostream&, const Matrix&)':
foo.cc:50:22: error: 'i' was not declared in this scope
   50 |                 _for(i,0,mat.dim_x)
      |                      ^
foo.cc:50:17: error: '_for' was not declared in this scope
   50 |                 _for(i,0,mat.dim_x)
      |                 ^~~~

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