retrofitted arithmetic functions

ast/eval.go

@@ -89,6 +89,11 @@ func (in *Token) Eval(funcs FuncTable, vars VarTable) *Token {
                 inner = &Token{inner: inner}
             }
             res = funct.CallFunction(inner.Next, vars, funcs).Eval(funcs, vars)
+            if res == nil {
+                // function failed. logging is its responsibility
+                return nil
+            }
+
             res.Append(in.Next)
 
         } else {