Broader definitions than yours are in common use. You might find this classic paper: http://www.is.pku.edu.cn/~qzy/plan/lits/FundamentalConceptOfPL.pdf http://w

Broader definitions than yours are in common use. You might find this classic paper: http://www.is.pku.edu.cn/~qzy/plan/lits/FundamentalConceptOfPL.pdf http://www.is.pku.edu.cn/~qzy/plan/lits/FundamentalConceptOf... and this updated survey: http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf interesting.