{def lcs
 {def lcs.rec
  {lambda {:a :b :w}
   {if {or {< {W.length :a} 2} {< {W.length :b} 2} }
    then {W.rest :w}
    else {if {W.equal? {W.first :a} {W.first :b}}
          then {lcs.rec {W.rest :a} {W.rest :b} :w{W.first :a}}
          else {let { {:x {lcs.rec :a {W.rest :b} :w}}
                      {:y {lcs.rec {W.rest :a} :b :w}}
                    } {if {> {W.length :x} {W.length :y}}
                       then :x
                       else :y} }}}}}
 {lambda {:a :b}
  {lcs.rec :a# :b# #}}}
-> lcs

{lcs testing123testing thisisatest}
-> tsitest   // 23000ms