Document(
  Prefix(ex   <http://example.org/example#>)
  
  Group  (
    Forall ?x ?y  ?z  ( ?y=?z   :-  And ( ex:p(?x ?y)  ex:p(?x ?z) ) )

    ex:p(ex:a ex:b)
    ex:p(ex:a ex:c)
  ) 
)