Refactored to allow ConceptEvaluator

docs/Concept_properties.md

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+class Concept:
+ * name
+ 
+    human reading name of the concept. It is how the concept will be referenced
+ * is_builtin
+    
+    Simple flag to make the difference between user defined concept and builtins
+    
+  * is_unique
+  
+    To manage singleton. There should be only one instance of the concept if True
+  
+  * key
+  
+    It is the name of the concept, where all variables, or the obj are replaced by
+    __var__x (x is the number of the variable). It's use to quickly find concepts
+    with variables
+    
+  * id
+  
+    The key is the unique identifier for the class of concept. The concept may be
+    modified along its life, but its key will never change
+
+  * where
+    
+    preconditions for the variables of the concept. Body will be executed only 
+    if the where conditions on the properties are True 
+
+  * pre
+    
+    Execution context to verified before executing the context
+
+  * post
+    
+    Post conditions to verify, once the context is executed
+  
+  * obj
+    
+    main or principal subject of the concept
+  
+  * body
+  
+    Code or value of the concept
+    
+   * desc
+   
+     explanation of the concept
+    
+    * digest
+     
+        SHA 256 of all properties. It allows 
+     
+    * props
+    
+        other properties of the concept (obj is the first one)
+