/in/foo.cc: In function 'int main()':
/in/foo.cc:10:10: error: 'stack' is not a member of 'std'
   10 |     std::stack<psi> st;
      |          ^~~~~
/in/foo.cc:4:1: note: 'std::stack' is defined in header '<stack>'; did you forget to '#include <stack>'?
    3 | #include <utility>
  +++ |+#include <stack>
    4 | 
/in/foo.cc:10:19: error: expected primary-expression before '>' token
   10 |     std::stack<psi> st;
      |                   ^
/in/foo.cc:10:21: error: 'st' was not declared in this scope; did you mean 'std'?
   10 |     std::stack<psi> st;
      |                     ^~
      |                     std