We rely, that standard library have really effective `+=` and `+` operators
for `std::string` with any paired arguments: `std::string`, `const char *` and `char`.
So, that change just blocks `c = a + b` and `c += a` replacements,
but still replaces the `c = a + b + d` and `c += a + b` and longer (with more `+` operators).