text: #include <string.h>
  #include "helpers.h"
  #include "memory.h"
  
  char *hstrcat(char *s1, char *s2)
  {
      const size_t a = strlen(s1);
      const size_t b = strlen(s2);
      const size_t size_ab = a + b + 1;
      s1 = realloc_memory(s1, size_ab);
      memcpy(s1 + a, s2, b + 1);
      return s1;
  }