add doc on cascading write operations in cascading tutorial
@@ -91,6 +91,43 @@ The **proc** namespace is not accessible by the command line tool **kdb**, as it
The **spec** namespace is used to store metadata about keys and therefore Elektra handles the **spec** namespace differently to other namespaces. The following part of the tutorial is dedicated to the impact of the **spec** namespace on cascading lookups.
+## Write Operations and the cascading Namespace
+
+When a value is to be written to a cascading key, i.e. a key starting with '/', only cascading keys that resolve to some existing key may be used.
+
+For example,
+
+```sh
+kdb set /tests/tutorial/cascading/#0/current/cascading_write_test value
+# STDERR: Aborting: A cascading write to a non-existent key is ambiguous.
+# RET: 2
+
+kdb meta-set /tests/tutorial/cascading/#0/current/cascading_write_test metakey metavalue
+# STDERR: Aborting: A cascading write to a non-existent key is ambiguous.
+# RET: 2
+```
+
+will both fail, as no matching key exists.
+Since there are multiple hypothetical key names that would match the cascading name (keys of specific namespaces like user:, system:, ...) if they existed, it is not clear what the user intended and thus an error is produced.
+
+To make the previous two operations meaningful, a matching key in the user: namespace is created:
+
+```sh
+kdb set user:/tests/tutorial/cascading/#0/current/cascading_write_test value
+#> Create a new key user:/tests/tutorial/cascading/#0/current/cascading_write_test with string "value"
+```
+
+Now, the operations operate on a well-defined key and succeed this time:
+
+```sh
+kdb set /tests/tutorial/cascading/#0/current/cascading_write_test value
+#> Set string to "value"
+#> Using name user:/tests/tutorial/cascading/#0/current/cascading_write_test
+
+kdb meta-set /tests/tutorial/cascading/#0/current/cascading_write_test metakey metavalue
+#> Using name user:/tests/tutorial/cascading/#0/current/cascading_write_test
+```
+
## Override Links
The `spec` namespace is special as it can completely change how the cascading
