Mr. Copland was his own piano soloist. As the expression goes, he played like a composer. Mr. Austin, a child of our times, uses the most fashionable elements. Aleatoric music is that in which the instrumentalists have complete leeway and are supposed to finish the thoughts of the composer. Of course, jazz has made use of aleatory long before the name was invented.
Improvisation is one form of aleatory.
And the three soloists had plenty of chance to improvise. Their improvisations were more interesting technically than musically. Indeed, the chief interest was on how these three accomplished musicians handled the trumpet, the bass, the percussion. When Don Ellis went into a long, perfectly controlled and slow pianissimo trill, he had the whole horn section of the Philharmonic hanging over their stands to see how he achieved his effects. Barre Phillips, on bass, and Joe Cocuzzo, on percussion, received equal attention from their Philharmonic opposite numbers.