mkdocs/docs/live/examples/stmvalidate.evl

context State {
	
	// Check that state names consist
	// of a single character
	constraint OneCharacter {
		check: self.name.length() = 1
		message: "State names must be one character long"
	}

	// Check that state names are unique
	constraint Unique {
		check: State.all.select(s|s.name = self.name).size() = 1
		message: "Duplicate state name " + self.name
	}
}

context Transition {
	
	// Check that there are no two transitions
	// for the same pair of states
	constraint Unique {
		check: Transition.all.
			select(t|t.from = self.from and t.to = self.to).size() = 1

		message: "Duplicate transition from " + self.from.name +
			" to " + self.to.name
	}
}