Support subset of instances of nested abstract clafers

[mantkiew]

The following example gives a NullPointerException:

abstract Component
    abstract Port
    abstract ExceptionPort : Port
    allPorts -> Port *
    [ allPorts.dref = this.Port ]    // the exception is caused by "this.Port"

WinController : Component
    cmd : Port
    powerOut : ExceptionPort

The expected output is (as produced by AlloyIG):

WinController
  allPorts$1 -> powerOut
  allPorts$2 -> cmd
  powerOut
  cmd

This works nicely for concrete clafers:

Component 3
    Port 1..3
    allPorts -> Port *
    [ allPorts.dref = this.Port ]

produces, for example an instance:

Component
  Port
  Port$1
  allPorts -> Port
  allPorts$1 -> Port$1
Component$1
  Port$2
  Port$3
  allPorts$2 -> Port$2
  allPorts$3 -> Port$3
Component$2
  Port$4
  Port$5
  allPorts$4 -> Port$5
  allPorts$5 -> Port$4

[mantkiew]

It's a peculiar bug because the following works fine

abstract Component
    abstract Port
//    abstract ExceptionPort : Port
    allPorts -> Port *
    [ allPorts.dref = this.Port ]

WinController : Component
    cmd : Port
    powerOut : Port //ExceptionPort

[JLiangWaterloo]

Fixed.