A type for skolems that are generated during capture conversion. Capture conversion narrows the type of a tree whose type has wildcard arguments. A typical situation is a treet of typeC[_ >: L <: U] and an expected typeC[X] whereX is an instantiatable type variable. To be able to instantiateX, we cast the tree to typeX[$n.CAP] where$n is a fresh skolem type with underlying typeTypeBox[L, U].