In file included from /in/Compile/check.cpp:7:
/in/sokoban.hpp:284:17: error: use of undeclared identifier 'is_valid'
            if (is_valid(nx, ny, rows, cols, grid) && !visited[static_cast<size_t>(nx)][static_cast<size_t>(ny)]) {
                ^
1 error generated.
/in/Compile/Makefile:16: recipe for target 'main' failed
make: *** [main] Error 1