public int depth() {
		int n = 0;
		RuleContext p = this;
		while ( p!=null ) {
			p = p.parent;
			n++;
		}
		return n;
	}
What state invoked the rule associated with this context?
 The "return address" is the followState of invokingState
 If parent is null, this should be -1 this context object represents
 the start rule.
runtime/Java/src/org/antlr/v4/runtime/RuleContext.java