Added RET keyword

src/core/builtin_concepts.py

@@ -68,6 +68,7 @@ class BuiltinConcepts(Enum):
     EVAL_WHERE_REQUESTED = "eval where requested"  # to evaluate the where clause
     CONCEPT_VALUE_REQUESTED = "concept value requested"  # returns the body of the concept instead of the concept itself
     REDUCE_REQUESTED = "reduce requested"  # remove meaningless error when possible
+    EVAL_SUCCESS_REQUESTED = "Try to find a successful evaluation"  # PyhtonEvaluator tries combination until True is found
     NOT_A_SET = "not a set"  # the concept has no entry in sets
     WHERE_CLAUSE_FAILED = "where clause failed"  # failed to validate where clause during evaluation
     CHICKEN_AND_EGG = "chicken and egg"  # infinite recursion when declaring concept
@@ -78,6 +79,7 @@ class BuiltinConcepts(Enum):
     NOT_INITIALIZED = "not initialized"
     NOT_FOUND = "not found"  # when the wanted resource is not found
     FORMAT_INSTRUCTIONS = "format instructions"  # to express how to print the concept
+    NOT_IMPLEMENTED = "not implemented"  # instead of raise an error
 
     NODE = "node"
     GENERIC_NODE = "generic node"