Update reachablenumberprog.en.md

subjects/reachablenumberprog.en.md

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 ## reachable_number
 
+## **WARNING! VERY IMPORTANT!**
+
+For this exercise a function will be tested **with the exam own main**. However the student **still needs** to submit a structured program:
+
+This means that:
+
+- The package needs to be named `package main`.
+- The submitted code needs one declared function main(```func main()```) even if empty.
+- The function main declared needs to **also pass** the `Restrictions Checker`(illegal functions tester). It is advised for the student to just empty the function main after its own testings are done.
+- Every other rules are obviously the same than for a `program`.
+
 ### Instructions
 
 Let us define a function f(x) by the following: first we add 1 to x, and then while the last digit of the number equals 0, we shall be deleting 0. Let us call 'y' reachable if we can apply **f** to **x** (zero or more times), and get **y**. 102 is reachable from 10098: f(f(f(10098))) = f(f(10099)) = f(101) = f(102). Any number is reachable from itself. You are given a positive number **n**, count how many integers are reachable from **n**.