assignment 1~5: fixes

- assignment05/pascal.mlw: lowered the difficulty (one more invariant given)
- assignment02, 03: minor fixes & divide into sub-problems

src/assignments/assignment03/parse_shell.rs

@@ -0,0 +1,28 @@
+//! Parsing a shell command.
+//!
+//! Shell commands are text-based instructions that you can enter in a command-line interface (CLI)
+//! to interact with operating systems (e.g. Linux) and others.
+//! For example, you can use the `ls` command to list files in a directory.
+//!
+//! You will parse a given string consists of a small number of shell commands.
+
+/// Parse the string as a shell command.
+///
+/// Usually, a shell command is whitespace-separated array of strings.
+/// ```text
+/// cat file  -->  ["cat", "file"]
+/// ```
+/// But sometimes, you may want to include whitespaces in each argument.
+/// In that case, you can use quotes.
+/// ```text
+/// ls 'VirtualBox VMs'  -->  ["ls", 'VirtualBox VMs']
+/// ls VirtualBox' 'VMs  -->  ["ls", 'VirtualBox VMs']
+/// ```
+///
+/// For simplicity, you may assume that the string only contains alphanumeric characters, spaces
+/// (" "), and single quotes ("'").
+///
+/// See `test_shell` for more examples.
+pub fn parse_shell_command(command: &str) -> Vec<String> {
+    todo!()
+}