+i.e. source → subtract position → lookup destination → add position
+
+---
+
+# Advance the wheel
+
+Trivial...
+
+# Encipher a letter
+
+Advance the wheel, then look up the letter
+
+---
+
+# Encipher a message
+
+```python
+ciphertext = ''
+for letter in plaintext:
+ ciphertext += encipher_letter(letter)
+return ciphertext
+```
+
+Have to be explicit as the order of the operations is important
+
+* Something like `map` might choose an order different from strict left-to-right
+
+## Test it against the physical object