VariablesAndKeywords

In IDL you can have variables and keywords like this:

[@pro test, variable, keyword=keyword@]

you can then call the procedure test in the following ways:
* [@test, 1@]: then 'variable' is set to 1 and keyword to 0 (i.e. not set)
* [@test, 1, /keyword@]: then 'variable' and keyword are set to 1
* [@test, 1, keyword=5@]: then 'variable' is set to 1 and keyword is set to 5