added Erathostene's sieve model

+# Erathostenes' sieve (https://mcc.lip6.fr/pdf/Eratosthenes-form.pdf)
+        
+lang coffee
+
+net "Erathostene sieve" :
+    place p number = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
+    trans t n % d == 0 :
+        < p var = n
+        < p var = d
+        > p var = d

+# Erathostenes' sieve (https://mcc.lip6.fr/pdf/Eratosthenes-form.pdf)
+        
+lang go
+
+net "Erathostene sieve" :
+    place p int = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
+    trans t n % d == 0 :
+        < p var = n
+        < p var = d
+        > p var = d

+# Erathostenes' sieve (https://mcc.lip6.fr/pdf/Eratosthenes-form.pdf)
+        
+lang python
+
+net "Erathostene sieve" :
+    place p int = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
+    trans t n % d == 0 :
+        < p var = n
+        < p var = d
+        > p var = d

@@ -30,7 +30,8 @@ class CodeGenerator (ast.CodeGenerator) :
         self.write(preamble % {"timestamp" : self.timestamp(),
                                "package" : node.name})
         self.children_visit(node.body)
-        self.fill(closing)
+        if node.name == "main" :
+            self.fill(closing)
     def _tupledef (self, typ) :
         return "_".join([""] + [_letter.sub("", self.typedef[t]) for t in typ] + [""])
     def _typedef (self, typ, place) :