Inserts a string into another string at position. If the position is
greater than the length of the string, the string will be padded with
blanks on the right.
Example:
St = 'ABCDEF'
Insert('123',St,4) ;returns 'ABC123DEF'
St = 'ABCDEF'
Insert('123',St,8) ;returns 'ABCDEF 123'
