Joost Noppen Pim van den Broek Nathan Weston Awais Rashid Modelling Imperfect Product Line Requirements with Fuzzy Feature Diagrams. 93-102 2009 VaMoS http://www.vamos-workshop.net/proceedings/VaMoS_2009_Proceedings.pdf conf/vamos/2009 db/conf/vamos/vamos2009.html#NoppenBWR09