glinear updates aren't allowed in ghost context

[tjhance]

glinear updates need to be allowed within ghost contexts:

glinear datatype F = F

glinear method maybe_swap(glinear x: F, glinear y: F, ghost b: bool)
returns (glinear x': F, glinear y': F)
{
  if b {
    x' := y;
    y' := x;
  } else {
    x' := x;
    y' := y;
  }
}

This should be allowed but currently errors with

GLinearGhost.dfy(10,7): Error: Assignment to non-ghost variable is not allowed in this context (because this is a ghost method or because the statement is guarded by a specification-only expression)
GLinearGhost.dfy(11,7): Error: Assignment to non-ghost variable is not allowed in this context (because this is a ghost method or because the statement is guarded by a specification-only expression)
GLinearGhost.dfy(13,7): Error: Assignment to non-ghost variable is not allowed in this context (because this is a ghost method or because the statement is guarded by a specification-only expression)
GLinearGhost.dfy(14,7): Error: Assignment to non-ghost variable is not allowed in this context (because this is a ghost method or because the statement is guarded by a specification-only expression)