Add guidance on using square brackets for variable-operations (#7)

iamc/variable.rst

@@ -21,16 +21,22 @@ A variable name should adhere to the following conventions:
 
 *  A *|* (pipe) character indicates levels of hierarchy.
 *  Do not use spaces before and after the *|* character, but add a
-   space between words (e.g., *Primary Energy|Non-Biomass Renewables*).
+   space between words
+
+   .. code:: yaml
+
+      Primary Energy|Non-Biomass Renewables
+
 *  Do not use abbreviations (e.g, *PHEV*) unless strictly necessary.
 *  Do not use abbreviations of statistical operations (*min*, *max*,
    *avg*) but always spell out the term.
 *  All words must be capitalised (except for *and*, *w/*, *w/o*, etc.).
-*  Add hierarchy levels where it might be useful in the future, e.g.,
-   use *Electric Vehicle|Plugin-Hybrid* instead of *Plugin-Hybrid
-   Electric Vehicle*.
-*  Do not include words like *Level* or *Quantity* in the variable,
-   because this should be clear from the context or unit.
+*  If necessary to add a method or operation-identifier (e.g., Share, per capita)
+   to a variable name, add it in square brackets, e.g.,
+
+   .. code:: yaml
+
+      Population|Urban [Share]
 
 Units
 -----