RosettaCodeData/Task/Longest-common-substring/Bracmat/longest-common-substring.bracmat

  ( lcs
  =   X a b x L
    .   !arg:(?a.?b)
      & 0:?L
      & :?X
      & ( @( !a
           :   ?
               ( ?x
               & @(!x:? [>!L)
               & @(!b:? !x ?)
               & @(!x:? [?L:?X)
               )
               (?&~)
           )
        | !X
        )
  )
& out$(lcs$(thisisatest.testing123testing))