Fix subject for reversebits from a program to a function

subjects/reversebits.en.md

@@ -1,21 +1,51 @@
 ## reversebits
 
+## **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
 
-Write a program that takes a `byte` in binary format, that reverses it `bit` by `bit` (as shown in the
-example) and that prints the result.
+Write a function that takes a `byte` in binary format, that reverses it `bit` by `bit` (as shown in the example) and returns the result
+
+### Expected function
+
+```go
+func ReverseBits(by byte) byte {
+
+}
+```
+### Usage
+
+Here is a possible [program](TODO-LINK) to test your function :
+
+```go
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	a := byte(0x26)
+	fmt.Printf("%08b\n", a)
+	fmt.Printf("%08b\n", ReverseBits(a))
+}
+```
 
 ### Expected output
 
 ```console
 student@ubuntu:~/piscine-go/reversebits$ go build
 student@ubuntu:~/piscine-go/reversebits$ ./reversebits
-Not enough arguments.
-student@ubuntu:~/piscine-go/reversebits$ ./reversebits 00100110 | cat -e
-01100100$
-student@ubuntu:~/piscine-go/reversebits$ ./reversebits "djs"
-The argument "djs" does not represent a byte
-student@ubuntu:~/piscine-go/reversebits$ ./reversebits "0102039s" | cat -e
-The argument "0102039s" does not represent a byte$
+00100110
+01100100
 student@ubuntu:~/piscine-go/reversebits$
 ```