TIP 475 [L1 ] proposes a new [string insert] command.
string insert string index insertString
Returns a copy of string with insertString inserted at the indexth character. index may be specified as described in the STRING INDICES' [L2 ] section.
If index is start-relative, the first character inserted in the returned string will be at the specified index. If index is end-relative, the last character inserted in the returned string will be at the specified index.
If index is at or before the start of string (e.g., index is 0), insertString is prepended to string. If index is at or after the end of string (e.g., index is end), insertString is appended to string.