NCic.metasenv -> NCic.substitution -> NCic.context ->
NCic.term -> NCic.term ->
NCic.metasenv * NCic.substitution
(* this should be moved elsewhere *)
NCic.metasenv -> NCic.substitution -> NCic.context ->
NCic.term -> NCic.term ->
NCic.metasenv * NCic.substitution
(* this should be moved elsewhere *)