diff --git a/src/bqn/rollim.bqn b/src/bqn/rollim.bqn
index 48919b3..d8f9ea3 100644
--- a/src/bqn/rollim.bqn
+++ b/src/bqn/rollim.bqn
@@ -60,3 +60,5 @@ HS ← +´∨≥1+⊒˜
 HL‿HS {𝕎•_timed𝕩}¨< 1e8 •rand.Range 1e3
 
 (+´⊢-˜⌈`⌾⌽⌊⌈`) [0,1,0,2,1,0,1,3,2,1,2,1]
+
+(+´⊢-˜⌈`⌾⌽⌊⌈`) [1,8,6,2,5,4,8,3,7]
diff --git a/src/rollim.org b/src/rollim.org
index dd16300..acd3c5c 100644
--- a/src/rollim.org
+++ b/src/rollim.org
@@ -274,8 +274,8 @@ I initially clip my citations array with ={≠¨⊔≠∘𝕩¨⌾(≥⟜≠∘
 
 ** Trapping rain water
 
-This problem is a classic in the realm of algorithms, and it can be solved in linear time. Interestingly,
-it admits a very elegant array solution as well:
+This is a classical interview problem that can be solved in linear time. Interestingly,
+it admits a very elegant array solution:
 
 #+begin_src bqn :tangle ./bqn/rollim.bqn :exports both
   (+´⊢-˜⌈`⌾⌽⌊⌈`) [0,1,0,2,1,0,1,3,2,1,2,1]
@@ -284,9 +284,16 @@ it admits a very elegant array solution as well:
 #+RESULTS:
 : 6
 
-That is, we do a max scan from the left and from the right, and then take the minimum of the two,
-then we subtract the height of the building at that point. The sum of all these values is the answer.
-A closely related problem is [[https://leetcode.com/problems/container-with-most-water/][container with most water]], which can be solved:
+That is, we take the minimum of max scans from the left and from the right, and subtract the corresponding height.
+Folding sum over the result gives the answer. A closely related problem is [[https://leetcode.com/problems/container-with-most-water/][container with most water]], which can
+be solved in linear time as well:
+
+#+begin_src bqn :tangle ./bqn/rollim.bqn :exports both
+  (+´⊢-˜⌈`⌾⌽⌊⌈`) [1,8,6,2,5,4,8,3,7]
+#+end_src
+
+#+RESULTS:
+: 19
 
 [fn:1] Almost Perfect Artifacts Improve only in Small Ways: APL is more French than English,
 Alan J. Perlis (1978). From [[https://www.jsoftware.com/papers/perlis78.htm][jsoftware]]'s papers collection.