+                                       #TODO: see if we can't prevent key normalisation with ->normalizeKeys(false)
+                                       #->scalarNode('roots')->cannotBeEmpty()->defaultValue($defaults['roots'])->end()
+                                       ->variableNode('roots')
+                                               ->treatNullLike([])
+                                               ->defaultValue($defaults['roots'])
+                                               #->scalarPrototype()->end()
+                                       ->end()