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subsection {* Policy Core *}
theory 
  PolicyCore
imports NetworkCore 
  "$AFP/UPF/UPF"
begin


text{* A policy is seen as a partial mapping from packet to packet out. *}

type_synonym ('\<alpha>, '\<beta>) FWPolicy = "('\<alpha>, '\<beta>) packet \<mapsto> unit" 

text{*
  When combining several rules, the firewall is supposed to apply the
  first matching one. In our setting this means the first rule which
  maps the packet in question to @{text "Some (packet out)"}. This is
  exactly what happens when using the map-add operator (@{text "rule1
  ++ rule2"}). The only difference is that the rules must be given in
  reverse order. 
*}



text{*   
  The constant @{text p_accept} is @{text "True"} iff the policy
  accepts the packet. 
*}

definition
  p_accept :: "('\<alpha>, '\<beta>) packet \<Rightarrow> ('\<alpha>, '\<beta>) FWPolicy \<Rightarrow> bool" where
  "p_accept p pol =  (pol p = \<lfloor>allow ()\<rfloor>)"

end