Add MentalExistent hierarchy of classes.

These differ from the MentalEntity class hierarchy in that
MentalExistents are a narrower subset of MentalEntities. MentalEntities
refer to all mental content that needs to be persistently stored to
represent cognitive and personna state.

MentalExistents are content of the mind that specifically represents
existents. I.e: perceptual data, whether structural or implicative.

smocore/include/mentalExistent.h

@@ -0,0 +1,37 @@
+#ifndef _MENTAL_EXISTENT_H
+#define _MENTAL_EXISTENT_H
+
+#include "mentalEntity.h"
+
+namespace smo {
+
+/*
+ * MentalExistents are content of the mind that specifically represents
+ * existents. I.e: perceptual data, whether structural or implicative. All
+ * Mentexents are Mentities, but not all Mentities are Mentexents.
+ */
+
+class MentalExistent
+: public MentalEntity
+{
+};
+
+typedef MentalExistent Mentexent;
+
+class ImplexedMentalExistent
+: public MentalExistent
+{
+};
+
+typedef ImplexedMentalExistent Implexent;
+
+class ArtificedMentalExistent
+: public MentalExistent
+{
+};
+
+typedef ArtificedMentalExistent Artifixent, Artixent;
+
+} // namespace smo
+
+#endif