The active element for method invocations. For
node are the receiver types. When a new receiver type is added, a new callee might be added.
Adding a new callee means linking the type flow of the actual parameters with the formal
parameters of the callee, and linking the return value of the callee with the return value
state of the invocation.
Statically bindable methods calls (
calls) have only one callee, but use the same code for