Probably the same number of arranger players are equally fooled into thinking that they can play piano, too. All evidence to the contrary...
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Perhaps, if thinking that one can play an arranger, while one CAN play the piano is an erroneous proposition, the reverse should be a given?
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Personally, I think that if one HAS the technique to play a piano well, all that needs to be applied is effort at understanding the operation of the arranger. However, the converse is not necessarily the case...