solve codewars 7-kyu a rule of divisibiliy by 7

codewars/7-kyu/a-rule-of-divisibility-by-7/solution.rkt

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+#lang racket
+
+;; https://www.codewars.com/kata/55e6f5e58f7817808e00002e
+
+(provide seven)
+
+(define (seven m)
+  (define (impl cur steps)
+    (define x (quotient cur 10))
+    (define y (modulo cur 10))
+    (define next (- x (* 2 y)))
+    (if (< cur 100)
+        (cons cur steps)
+        (impl next (+ 1 steps))))
+  (impl m 0))