let label_of_kind = function
| PKProp -> "Proof Obligation"
| PKPropLoop -> "Invariant as hypothesis" (* should be assert false ??? *)
| PKEstablished -> "Establishment"
| PKPreserved -> "Preservation"
| PKVarDecr -> "Decreasing"
| PKVarPos -> "Positive"
| PKAFctOut -> "Function assigns"
| PKAFctExit -> "Exit assigns"