diff --git a/src/mu.gmo b/src/mu.gmo
index 4375a39..0abd7e1 100644
--- a/src/mu.gmo
+++ b/src/mu.gmo
@@ -3,3 +3,26 @@
 > Mu is a minimal-dependency hobbyist computing stack 
 
 => https://github.com/akkartik/mu mu repository
+=> http://akkartik.name/akkartik-convivial-20200607.pdf bicycles for the mind have to be see-through
+
+# notes / runbook
+
+## some errors
+
+when there was a mismatch between a function header return register, and the register where the function call was performed:
+
+```
+register for output 'x' is not right
+```
+
+example code that triggers it:
+
+```
+fn f -> _/eax: int {
+  return 3
+}
+
+fn main {
+  var x/ecx: int <- f
+}
+```