diff --git a/src/references.gmo b/src/references.gmo
index d567c37..55faee1 100644
--- a/src/references.gmo
+++ b/src/references.gmo
@@ -50,10 +50,13 @@ texts by solderpunk in the {gemini} protocol
 => https://www.turingtumble.com/ turing tumble
 => https://www.youtube.com/watch?v=rA5qnZUXcqo The Stilwell Brain - Vsauce (youtube)
 => https://esoteric.codes/blog/escher-circuits-using-vision-to-perform-computation Escher Circuits: Using Vision to Perform Computation
-=> https://www.gwern.net/docs/www/think-maths.co.uk/5d2586e9724de4c5df5b354ee6117fcc2e1b722b.pdf Logic Gates - Domino Computer
-=> https://www.omanobserver.om/article/44559/Business/aba-oman-students-set-world-record-for-dominoes-circuit ABA Oman students set world record for dominoes circuit
+=> https://groups.csail.mit.edu/mac/users/bob/sliding-blocks.pdf The Complexity of Sliding-Block Puzzles and Plank Puzzles - Robert A. Hearn
+
+## fluidics-related
 
 => ./fluidics.gmi {fluidics}
+=> https://www.gwern.net/docs/www/think-maths.co.uk/5d2586e9724de4c5df5b354ee6117fcc2e1b722b.pdf Logic Gates - Domino Computer
+=> https://www.omanobserver.om/article/44559/Business/aba-oman-students-set-world-record-for-dominoes-circuit ABA Oman students set world record for dominoes circuit
 
 # computer science
 
@@ -63,6 +66,7 @@ texts by solderpunk in the {gemini} protocol
 => https://www.youtube.com/watch?v=xP5-iIeKXE8 A video of Conway's Game of Life, emulated in Conway's Game of Life.
 => https://aphyr.com/posts/341-hexing-the-technical-interview Hexing the technical interview
 => https://tromp.github.io/cl/diagrams.html lambda diagrams
+=> http://www.martin-gardner.org/SciAm1.html#1 Martin Gardner and Scientific American
 
 ## studies