Concepts bodies are now evaluated on demand

_concepts.txt

@@ -40,6 +40,13 @@ eighteen isa number
 nineteen isa number
 twenty isa number
 def concept twenties from bnf twenty number where number < 10 as twenty + number
+twenties isa number
 def concept thirty as 30
+def concept thirties from bnf thirty number where number < 10 as thirty + number
+thirties isa number
 def concept forty as 40
-
+def concept forties from bnf forty number where number < 10 as forty + number
+forties isa number
+def concept fifty as 50
+def concept fifties from bnf fifty number where number < 10 as fifty + number
+fifties isa number