tree←1(2(4(7⍬⍬)⍬)(5⍬⍬))(3(6(8⍬⍬)(9⍬⍬))⍬)
visit←{⍺,(×≢⍵)⍴⊃⍵}
children←{⊂¨@(×∘≢¨)1↓⍵}