diff --git a/core b/core
index ccbb516..a7cc80a 160000
--- a/core
+++ b/core
@@ -1 +1 @@
-Subproject commit ccbb5168b79c1b349159bc4a3c6e581d33056466
+Subproject commit a7cc80af75b33c555e7f0f1e552483303c188ccd
diff --git a/doc/OBSERVATIONS.md b/doc/OBSERVATIONS.md
index 9f2717e..60fa8ca 100644
--- a/doc/OBSERVATIONS.md
+++ b/doc/OBSERVATIONS.md
@@ -31,3 +31,12 @@
 ```
 
 `u/optimize` generates and judges candidate prompts, emitting the best to stdout; `b/q` acts on it. Any tool that reads stdin and writes stdout is already a pipeline stage.
+
+**Prompt composition** — `s/bfg` outputs can be nested to arbitrary depth using shell command substitution. The inner prompt completes, its output becomes part of the outer prompt:
+
+```
+; echo 'translate to japanese: '`echo 'say hello' | s/bfg` | s/bfg
+こんにちは！👋
+```
+
+This enables prompt pipelines where each stage transforms, filters, or augments the previous stage's output — all with standard shell syntax.