/in/foo.cc: In function 'int hw(std::string&, int)':
/in/foo.cc:5:26: error: conversion from 'void' to non-scalar type 'std::string' {aka 'std::__cxx11::basic_string<char>'} requested
    5 |         string s3=reverse(s4.begin(),s4.end());
      |                   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:23:19: error: 'substr' was not declared in this scope; did you mean 'strstr'?
   23 |         string s2=substr(i,i+j);
      |                   ^~~~~~
      |                   strstr