Update 3.1-frontend.md

src/3.1-frontend.md

@@ -9,4 +9,10 @@ To support project-level program analysis, we want the analysis tool to be integ
 ```
 cargo toolname -more_arguments
 ```
-Cargo will automatically search the binaries named cargo-toolname from the paths.
+Cargo will automatically search the binaries named cargo-toolname from the paths. The following figure demonstrates the whole process before reaching our analysis program.
+![Workflow of how cargo dispatches the analysis command to rap.][figure/cargoflow.png]
+Note that we cannot directly invoke rap in the first round but through cargo check because we need cargo to manage the project-level compilation and append detailed compilation options for launching rustc. However, we want to hook rustc execution and execute rap instead for analysis. Therefore, we set [RUSTC_WRAPPER](https://doc.rust-lang.org/cargo/reference/environment-variables.html) with the value of cargo-rap. In this way, cargo check will actually run `cargo-rap rustc appended_options`. We then dispath the execution to rap with appended options.
+
+## Register Analysis Callbacks
+
+