1 files changed, 29 insertions, 0 deletions
diff --git a/help/markfirst_memberof.md b/help/markfirst_memberof.md
new file mode 100644
index 00000000..228d6a50
--- /dev/null
+++ b/help/markfirst_memberof.md
@@ -0,0 +1,29 @@
+*View this file with results and syntax highlighting [here](https://mlochbaum.github.io/BQN/help/markfirst_memberof.html).*
+
+# Element Of (`∊`)
+    
+## `∊ 𝕩`: Unique Mask
+    
+Mark the first occurrence of each major cell in `𝕩` with a 1, and all other occurrences with a 0.
+    
+           ∊ 4‿5‿6‿6‿4‿7‿5
+
+           a ← 3‿3 ⥊ ↕9
+
+           ∊ a
+
+    
+    
+## `𝕨 ∊ 𝕩`: Member Of
+    
+Is each element in `𝕨` a major cell of `𝕩`?
+    
+           ⟨1⟩ ∊ ↕9
+
+           a ← 3‿3 ⥊ ↕9
+
+           ⟨0‿1‿2⟩ ∊ a
+
+           ⟨1‿3 ⥊ 0‿1‿2⟩ ∊ a
+
+