Add book section on controlling the number of operations generated (#246)

* doc: add book section on controlling number of operations generated

* Update book/src/features/structured-testing.md

Co-authored-by: Cameron Bytheway <[email protected]>

---------

Co-authored-by: Cameron Bytheway <[email protected]>

Author: mkforsb

book/src/features/structured-testing.md

@@ -59,7 +59,50 @@ fn main() {
 }
 ```
 
-This basic test will make sure we don't panic on any of the list of operations. We can take it to the next step by using a test oracle to make sure the behavior of `MySet` is actually correct. Here we'll use `HashSet` from the `std` library:
+## Controlling The Number Of Operations Generated
+
+Using `check!().with_type::<Vec<T>>()` will generate vectors with lengths in the range `0..=64`. If you need more control over the number of elements being generated you can instead use `.with_generator` and provide a customized generator:
+
+```rust
+# use bolero::{check, generator::*};
+use bolero::gen;
+# use my_set::MySet;
+
+# #[derive(Debug, TypeGenerator)]
+# enum Operation {
+#     Insert(u64),
+#     Remove(u64),
+#     Clear,
+# }
+#
+fn main() {
+    check!()
+        // Generate 0 to 200 operations
+        .with_generator(gen::<Vec<Operation>>().with().len(0..=200))
+        .for_each(|operations| {
+#             let mut set = MySet::new();
+#
+#             for operation in operations.iter() {
+#                 match operation {
+#                     Operation::Insert(value) => {
+#                         set.insert(value);
+#                     }
+#                     Operation::Remove(value) => {
+#                         set.remove(value);
+#                     }
+#                     Operation::Clear => {
+#                         set.clear();
+#                     }
+#                 }
+#             }
+            // assertions go here
+        })
+}
+```
+
+## Using Test Oracles
+
+The basic test we constructed above will make sure we don't panic on any of the list of operations. We can take it to the next step by using a test oracle to make sure the behavior of `MySet` is actually correct. Here we'll use `HashSet` from the `std` library:
 
 ```rust
 use bolero::{check, generator::*};