I wasnt implying an atonal 12-note approach.I was suggesting that "what are the chords for Fields of Athenry" implies an approach where a tune has a given agreed harmonic structure. That assumption bound together early jazz performances, where the harmonic structure, shared by all the musicians playing together, determined how each musician improvised, and so held the performance together. Miles Davis in later years got away from this, favouring a more Eastern or "monodic folk" approach where the mode or scale dominated the thinking. This givesyou a freedom to construct melodic phrases in quite a different way, as you are not constrained by feeling " everything I play in this bar has got to fit with a C7 chord".