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Digital root is the recursive sum of all the digits in a number.

Given n, take the sum of the digits of n. If that value has more than one digit, continue reducing in this way until a single-digit number is produced.
This is only applicable to the natural numbers.

Examples
    16  -->  1 + 6 = 7
   942  -->  9 + 4 + 2 = 15  -->  1 + 5 = 6
132189  -->  1 + 3 + 2 + 1 + 8 + 9 = 24  -->  2 + 4 = 6
493193  -->  4 + 9 + 3 + 1 + 9 + 3 = 29  -->  2 + 9 = 11  -->  1 + 1 = 2





def digital_root(number)
    #TODO
    return





Test.assert_equals(digital_root(16), 7)
Test.assert_equals(digital_root(942), 6)
Test.assert_equals(digital_root(132189), 6)
Test.assert_equals(digital_root(493193), 2)