docs: think more about design

DESIGN.md

@@ -12,12 +12,14 @@ basic design:
 ```python
 from swydd import taks, option, cli
 
+
 @task
 @option("program", "name of program to compile")
 def build(program: str = "hello"):
     """build a program"""
     sub < f"gcc -o {program} {program}.c"
 
+
 cli()
 ```
 
@@ -32,11 +34,10 @@ cli()
 
 ```python
 @task
-def pipe_commands()
+def pipe_commands():
     """run pre-commit (and mypy)"""
     p = Pipe(Exec("find . -name file"), Exec("grep 'pattern'")).get()
     print(p.stdout)
-
 ```
 
 Made even simpler with operator overloading:
@@ -61,12 +62,23 @@ def run_commands():
     print(stdout)
 ```
 
+## Handling a DAG
+
+```python
+@task
+@needs("src/a-file.txt")
+@targets("build/output.txt")
+def build():
+    """build step with a dependency"""
+    lines = asset("src/a-filter.txt").read().splitlines()
+    # or lines = ctx.needs[0].read().splitlines()
+    ctx.targets.output.write("\n".join([f"{i}: {line}" for i, line in enumerate(lines)]))
+```
+
+
+
 ## Internal CLI
 
 ```sh
-./tasks.py _ swydd-subcmd
+./tasks.py +swydd --version # <- current favorite (use a none valid function name to possibly prevent overlap)
-# vs
-./tasks.py _swydd-subcmd
-# or
-./tasks.py +swydd subcmd # <- current favorite (use a none valid function name to possibly prevent overlap)
 ```