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+# Test driven development (TDD)
+
+The suggested technique for bruijn development is the TDD method. When
+creating functions, we suggest the following procedure:
+
+-   Write a comment, a type signature, and the head of the function
+
+``` bruijn
+# measures the length of a list
+length y [[[rec]]] (+0) ⧗ (List a) → Number
+```
+
+-   Write several tests including all edge cases
+
+``` bruijn
+# measures the length of a list
+length y [[[rec]]] (+0) ⧗ (List a) → Number
+
+:test (length empty) ((+0))
+:test (length "a") ((+1))
+:test (length ({}empty)) ((+1))
+:test (length (empty : {}empty)) ((+2))
+:test (length ("abc")) ((+3))
+```
+
+-   Finish the implementation until all tests pass (e.g. using the
+    [`:watch`{.bruijn} repl command](REPL.md#watch))
+-   Refactor and clean up the definition
+
+``` bruijn
+# measures the length of a list
+length y [[[rec]]] (+0) ⧗ (List a) → Number
+    rec 0 [[[case-inc]]] case-end
+        case-inc 5 ++4 1
+        case-end 1
+
+:test (length empty) ((+0))
+:test (length "a") ((+1))
+:test (length ({}empty)) ((+1))
+:test (length (empty : {}empty)) ((+2))
+:test (length ("abc")) ((+3))
+```