FIBOFTF — isAssignable incorrect domain and missing label

The restriction on the property 'isAssignable' has a contract-specific definition but is given a domain of Thing, with a restriction replacing this definition for Contract itself.

Since the concept of being assignable only has this meaning with regard to contracts of one sort or another, there is no clear value in replacing this property with a restriction. Nor is there any more general kind of assignability concept of which the contract-specific one is a restriction. The definition in place for this property is already the contract-specific definition from the original model, so that did not need to change (but would have been wrong when the domain was Thing).

Review whether the restriction is still needed 9it has the function of stating that a contract always has the property of isAssignable meaning in the open world that a contract is always either assignable or not assignable, which is true.

Add domain of Contract to isAssignable.