Allow empty functions in Function problem

Support boolean events.

examples/experiments/Function.rb

@@ -2,7 +2,7 @@ module Function
 
   def problem(state, *args)
     function = state[:function] = {}
-    state.delete('function').each {|f,v| function[f] = v.to_f}
+    state.delete('function')&.each {|f,v| function[f] = v.to_f}
     state['protect_axiom'] ||= []
     super
   end