First version of explain. Creating a new parser was a wrong approach. Need to reimplement

src/core/builtin_concepts.py

@@ -58,6 +58,7 @@ class BuiltinConcepts(Enum):
     WHERE_CLAUSE_FAILED = "where clause failed"  # failed to validate where clause during evaluation
     CHICKEN_AND_EGG = "chicken and egg"  # infinite recursion when declaring concept
     ISA = "is a"  # builtin concept to express that a concept is an instance of another one
+    EXPLANATION = "explanation"
 
     NODE = "node"
     GENERIC_NODE = "generic node"
@@ -436,3 +437,17 @@ class NotForMeConcept(Concept):
 
     def __repr__(self):
         return f"NotForMeConcept(source={self.body}, reason={self.get_prop('reason')})"
+
+
+class ExplanationConcept(Concept):
+    def __init__(self, digest=None, command=None, title=None, instructions=None, execution_result=None):
+        super().__init__(BuiltinConcepts.EXPLANATION,
+                         True,
+                         False,
+                         BuiltinConcepts.EXPLANATION)
+        self.def_prop("digest", digest)  # event digest
+        self.def_prop("command", command)  # explain command parameters
+        self.def_prop("title", title)  # a title to the explanation
+        self.def_prop("instructions", instructions)  # instructions for SheerkaPrint
+        self.set_metadata_value(ConceptParts.BODY, execution_result) # list of results
+        self.metadata.is_evaluated = True