  A permutation is (1-23-4)-avoiding if it contains no four entries, increasing
left to right, with the middle two adjacent in the permutation. Here we give a
2-variable recurrence for the number of such permutations, improving on the
previously known 4-variable recurrence. At the heart of the proof is a
bijection from (1-23-4)-avoiding permutations to increasing ordered trees whose
leaves, taken in preorder, are also increasing.
