SEDOLs

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For each number list of 6-digit SEDOLs, calculate and append the checksum digit. That is, given the input string on the left, your function should return the corresponding string on the right:

710889 => 7108899 B0YBKJ => B0YBKJ7 406566 => 4065663 B0YBLH => B0YBLH2 228276 => 2282765 B0YBKL => B0YBKL9 557910 => 5579107 B0YBKR => B0YBKR5 585284 => 5852842 B0YBKT => B0YBKT7 B00030 => B000300

Check that each input is correctly formed, especially with respect to valid characters allowed in a SEDOL string. Your function should return null on an invalid input.