Another divisibility rule for 7

Separate the candidate number as usual, into groups of two digits starting from the end. For example, 10101 = 1 01 01. Starting from the beginning of the number, eliminate each group of two digits adding it twice to the next group. Continuing the example, 1 01 01 = 01 + 1*2 01= 03 01= 03*2 + 01=7. The result shows the modulo 7 result of the candidate number. Here, 10101 modulo 7 = 7 = 0. Another example: 15113 = 1 51 13 = 51 +2*1 13 = 53 13 = 13+ 53*2 = 119 = 19 + 2 = 21 = 0 modulo 7. If results of an addition get too unwieldy, you can simplify at any moment processing the partial result itself as if it were a candidate number.