Cannot use $$ in &while on vector field

[bbannier]

I would expect that explicitly naming the current field or using $$ would be equivalent, but it is not.

module foo;

type X = unit {
    a: Y(self)[] &while=(|self.a| < 4711);  # WORKS
    b: Y(self)[] &while=(|$$| < 4711);      # ERROR: unsupported operator: |<Y>|
};

type Y = unit(x: X) {
    : b"\x00";
};