Moved tests and added comments

.gitignore

@@ -1 +1,3 @@
 /target
+.idea/
+*.iml

src/lib.rs

@@ -5,3 +5,83 @@ pub fn sum(num1: i32, num2: i32) -> i32 {
 pub fn mul(num1: i32, num2: i32) -> i32 {
     num1 * num2
 }
+
+/*
+Why the move?
+
+
+It's possible to locate unit tests outside the production-code files (although you'd still want
+to tag them with #[cfg(test)]), but that's not the convention in Rust, and that's not the way we
+do it in MASQ.
+
+Unit tests go in the same file as the code they test.
+
+That isn't to say we never have test/ directories. We do, but it's _integration_ tests that go in
+the test/ directories. Integration tests don't have the same kind of tightly-coupled access to the
+production code that unit tests do. For integration tests, the production code is processed into
+a separate crate, and the integration tests are forced to interact with it only through the public
+crate interface.
+
+That's quite useful--for integration tests--but you don't want to try writing unit tests that way.
+ */
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /*
+    Note:
+
+    I understand that tests are fun to write and even more fun to watch pass. In an exercise like
+    this, there's no reason not to write all the tests you want.
+
+    However, in real life, unless you're making a specific rhetorical point, you want to confine
+    yourself to tests that drive code. For example, you could drive either sum() or mul() by writing
+    one of the tests below. You'd write the test, watch it fail, complete either sum() or mul(),
+    watch the test succeed, and then...you're done.  You can write the other three tests if you
+    want, but they'll pass immediately: they won't drive any new code. Tests that don't drive code
+    are overhead: they increase the cost of refactoring, if you decide on a refactoring that requires
+    changing tests. That might be justified if the extra tests communicate something important to a
+    reader; otherwise usually not.
+     */
+
+    #[test]
+    fn sum_positives() {
+        assert_eq!(sum(3, 4), 3 + 4)
+    }
+
+    #[test]
+    fn sum_negatives() {
+        assert_eq!(sum(-2, -3), -2 + (-3))
+    }
+
+    #[test]
+    fn sum_positive_negative() {
+        assert_eq!(sum(2, -3), 2 + (-3))
+    }
+
+    #[test]
+    fn sum_negative_positive() {
+        assert_eq!(sum(-2, 3), -2 + 3)
+    }
+
+    #[test]
+    fn mul_positives() {
+        assert_eq!(mul(3, 4), 3 * 4)
+    }
+
+    #[test]
+    fn mul_negatives() {
+        assert_eq!(mul(-2, -3), -2 * (-3))
+    }
+
+    #[test]
+    fn mul_positive_negative() {
+        assert_eq!(mul(2, -3), 2 * (-3))
+    }
+
+    #[test]
+    fn mul_negative_positive() {
+        assert_eq!(mul(-2, 3), -2 * 3)
+    }
+}