Since first posting on the topic, I've now played (in my halting way) the solo piano ad lib introduction to the live Ethan Iverson/ Paul Motian/ Larry Grenadier performance of It's Easy to Remember in Guillaume Hazebrouck's transcription, and listened to it several more times.  I'm even more taken by this masterful performance, especially the introduction.  The harmonies in the introduction are often quite dissonant but beautifully limpid, probably due to the very open voicings (wide intervals), and choice of intervals.  The dissonances reminiscent of 20th century classical music combined with untypical but compelling voiceleading remind me a bit of Bill Evans, but the choice of intervals and limpid sonority doesn't so much.  The (incomplete) blow-by-blow that follows is mostly for my own reference, so you might skip down to the next paragraph if harmonic analysis doesn't interest you.  It's far from crucial for appreciating the music, but I really want to know how these sounds are made.  The first part is mostly over an E flat pedal (the piece is in E flat), with couple of excursions to Ab. The first chord is fabulous, with successive intervals of a minor ninth, minor 7th, minor 6th (Eb, E, D, Bb).  Then the two inner voices move inward by a half step for another open, somewhat dissonant chord.  It's perhaps not so important to analyze these harmonically, but the first comes off pretty clearly as an Eb major voicing, with no 3rd which no doubt contributes to the spare, clear sound, and with a major 7th, and as for the E natural (b9 you could say), well it just sounds great, and moves up to a natural 9 on the next chord, while the 7th moves down to the minor 7th of Eb, suggesting perhaps a change in quality to dominant or minor, though not this is not so clear as there's still no 3rd present.  Later in the introduction, the same voicing will indeed function as a dominant leading to an Ab major triad at the end of the first system of the transcription.  But first we get a repeat of the first two chords at a faster pace, except with A natural in place of Bb in the top voice (which is basically paraphrasing the melody).  The tenor voice is going up chromatically, cadencing toward a G as part of the double-whole-note Eb major 7th, the first time we get a 3rd with an Eb chord.  The repose is disturbed with a little tweak up to a B natural in the treble, just to add a little more pretty dissonance to the picture. (Nothing wrong with a touch of the "girlfriend chord" once in a while.)  Then we again get those first two chords, Bb in the treble again, moving in quarters, initiating the same four-quarter-note chromatic ascension in the tenor to G, but the bass moving up to Ab on the last two quarters, over which the harmony sounds first like Ab7, then Ab m7, while the top Bb leads down into a bluesy figure.  The next system finishes out with more chromatic movement in the bass, more intricate melody in the top voice accompanied by good inner voice action especially in the tenor, and a final cadence on Eb major again, with the 3rd but in the same open voicing that marked the first appearance of the G before, except that now the D forms a minor 2nd cluster with that seemingly outrageous, but beautiful, E natural, kind of fusing the initial two dissonant Eb voicings but with the added 3rd for an earthier, more harmonically grounded sound, perfectly capping off the introductory chorus.

Besides the open voicings and relatively spare use of 3rds (so that they are all the more effective when they are used), movement by half-steps is a major feature of the voice-leading in this introduction, but it doesn't come across with any feeling of slick hepness or angst-ridden compulsion, perhaps because it's not being used heavily as b9 or #11 over dominant chords, or in related diminished or augmented substitutions for dominants.  Maybe there is a relative absence of tritones in the voicings, though I didn't check carefully.  Anyway, the half-step motion is prominent enough to be considered a major musical ingredient, but doesn't really interfere with what sounds to me like a relatively diatonic, if sometimes beautifuly dissonant, feel.  I guess the chromatic motion is not, for the most part, setting up dissonances that cry out for an obvious resolution, nor effecting such resolution.  It reminds me a bit of Stravinksy in that the dissonance is often created by the interaction of natural melodic motions in the voices, and (along with the melodic motion) the actual intervals in the chord seem almost more important than any compulsive "functional" movement in the harmony even though there is some of the latter on occasion.

The other remarkable thing about Iverson's playing on this piece is the strong influence of Monk, assimilated well into Iverson's own style, in the trio portion of the piece.  Monkian upward arpeggios appear as early as measure 16 (the 3rd measure of the first trio chorus), often combined with scalar material that still sounds quite Monkish (as in measure 16), or leading into more original melodic figures (as in measures 25-26).  A classic downward-dropping Monk left-hand figure is used in measure 30, a very bluesy Monkian chorus-ending figure at 44-46, upward arpeggios in 47-48 lead again to more personal Iversonian material in 49-50, and the list could go on.  Often Iverson seems to be extending or filling in Monkish lines with his own material more reminiscent of more standard bop-influenced lines, but never quite the standard bop clichés.  There's lot's of great action in the inner voices too, sometimes Monkian, sometimes not particularly so.  I think Monk's vocabulary and approach, even while it contributed crucially to the lingua franca of bebop and beyond, has probably been underexploited by pianists who are perhaps rightly afraid that it's hard to make something personal this way, something that doesn't sound like copying Monk's licks, but Iverson makes it work to great effect.  (I guess you could argue that a few other pianists have been strongly influenced by Monk's approach while keeping the harmonic and melodic content of their playing further from Monk than Iverson does here.)

In fact, the display of constructive influence by Monk, and the use of Monkian influences in a clear personal style, makes me wonder if the introduction might be more influenced by Monk than I realized.  I haven't listened to Monk's solo piano for a while, and it is probably time to listen to more.

Speaking of more, here's hoping we get to hear more from this set, or others in the same week at the Vanguard.  All About Jazz's review of what was probably the first set on that same Friday (March 11, 2011) is tantalizing, too.  This is some of the most interesting piano playing I've heard in many years---jazz of the highest order.