NCic.metasenv -> NCic.substitution -> NCic.context ->
(NCic.term * NCic.term) -> (NCic.term * NCic.term) list ->
(NCic.term * NCic.term * NCic.metasenv * NCic.substitution) list
NCic.metasenv -> NCic.substitution -> NCic.context ->
(NCic.term * NCic.term) -> (NCic.term * NCic.term) list ->
(NCic.term * NCic.term * NCic.metasenv * NCic.substitution) list