coerce.java

(nat;attr);(nat;attr) => edge

((nat;attr);(nat;attr)), 
 (vertex;vertex), 
 [combine,(vertex<=(nat;attr)[split,('id'->nat<=(nat;attr)[get,0,_],
                                     'attrs'->attr<=(nat;attr)[get,1,_])];
           vertex<=(nat;attr)[split,('id'->nat<=(nat;attr)[get,0,_],
                                     'attrs'->attr<=(nat;attr)[get,1,_])])], 
 [split,('outV'->vertex<=(vertex;vertex)[get,0,_],
         'inV'->vertex<=(vertex;vertex)[get,1,_])], 
  edge<=(vertex;vertex)
  [split,('outV'->vertex<=(vertex;vertex)[get,0,_],
          'inV'->vertex<=(vertex;vertex)[get,1,_])]