rql #67696 crash with undo / exists node [validation pending]

rql

Any U,COUNT(P) GROUPBY U WHERE U is CWUser, P? patch_reviewer U, EXISTS(P in_state S, S name IN("in-progress", "pending-review"))

generate following traceback when facets goes in:

File /usr/lib/pymodules/python2.6/rql/undo.py, line 155, function undo
  assert isinstance(parent, Select)
prioritynormal
typebug
done in0.29.0
load0.200
load left0.000
closed by#e54b9379fc7c closes #67696: crash with undo/Exists nodes (unreproduced by test though :( )