# Compose a subsystem and inspect its impact

Prerequisite: [research records](research.md). A composition pins a set of recorded
spell versions as one named unit. Its identity follows its membership. Recompose
adds or removes selected members and produces a successor; it does not erase the
previous roster.

## Read the unit at the right grain

Roster, history, and member differences come from the research record. Footprint,
drift, and impact join the member versions to custody information. Missing custody
is reported explicitly; it must not silently count as an empty dependency graph.

The subsystem lesson creates three real module worlds before forming its group.
Impact considers the union of member effects and distinguishes effects inside the
composition from effects outside it. The membership comparison remains at
subsystem grain; select changed members for deeper code comparisons afterward.

## Compose source separately

`research_synthesize` selects top-level functions or classes from a donor module
into a base module and returns composed source with a preview. A method is not a
top-level function. The workshop lesson exercises both replacement and addition
using two generated modules so the comparison has two genuinely different inputs.

Staged ancestry belongs to the next fresh world entry. If the candidate is
abandoned, clear that staging explicitly so an unrelated later bind cannot inherit it.


## The workflow in code

These are the core steps from the saved example. Use its complete linked script for all class definitions and setup.

```{literalinclude} ../downloads/04_expert/30_composing_a_version_in_the_workshop.py
:language: python
:pyobject: main
:linenos:
```


## Runnable examples

- [Composing a version in the workshop](../examples/expert/30-composing-a-version-in-the-workshop.md) — Expert 30
- [Compositions a subsystem as one unit](../examples/expert/14-compositions-a-subsystem-as-one-unit.md) — Expert 14
- [A subsystems blast radius](../examples/expert/32-a-subsystems-blast-radius.md) — Expert 32

[All expert examples](../examples/expert/index.md) · [Level contents](index.md) · [Full contents](../contents.md)

[Canonical page source](https://github.com/Synaptic724/melder/blob/90d52e0a12cb0743d62568d5a19ce954a54443bc/docs/expert/compositions.md)