Add function string_subset

m-string.h

@@ -709,6 +709,18 @@ string_mid (string_t v, size_t index, size_t size)
   string_left(v, size);
 }
 
+/* Set the string 'v' to the substring of org from position index to size bytes */
+static inline void
+string_subset (string_t v, const string_t org, size_t index, size_t size)
+{
+  // The substring shall fully exist within the string org
+  assert(index + size <= string_size(org) );
+  assert(v != org);
+  
+  const char *str = string_get_cstr(org);
+  string_set_strn(v, &str[index], size);
+}
+
 /* Return in the string the C string str1 into the C string str2 from start
    By default, start is zero.
 */