cycle detection of instances

[mathgladiator]

record X {
  public Y y;
}

record Y {
  public X x;
}

public X x;
public Y y;

[mathgladiator]

Since this is a direct field, we really need to build a "instance graph", so type R instances types r0, r1, ..., rN; this then needs to go into a cyclic graph algorithm to abort