Assertion: template's option | /r/HasProperty | set
Facets:
Source sentences:
text: While it's simple testing if a Boolean flag is set (simply use "elem"), it's harder to check if an option with an argument is set.
text: If you use a variable that doesn’t exist, the template system will insert the value of the string_if_invalid option, which is set to '' (the empty string) by default.