@prefix rdfs:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix log:  <http://www.w3.org/2000/10/swap/log#> .
@prefix string:  <http://www.w3.org/2000/10/swap/string#> .
@prefix : <#>.
@prefix v: <var#> .
@prefix doc: <http://infomesh.net/2001/09/doc/#> .

this log:forAll :x, :y, :z, :ax, :ay, :bx, :by .

# { :x string:concat ([ is log:resolvesTo of <a.txt>] 
#    [ is log:resolvesTo of <b.txt>]) }
# log:implies { :ab :is :x } .

# { :x string:concat ("a" "b") }
# log:implies { :x :is :x } .

{ <a.txt> log:resolvesTo [ log:includes { :ax doc:content :ay } ] .
  <b.txt> log:resolvesTo [ log:includes { :bx doc:content :by } ] }
log:implies { :x string:concat (:ay :by) } .

# { :x string:concat (:ay :by) }
# log:implies { :x :is :x } .