If it is legitimate to ask students (no questions asked) to double M/P (by cutting P in half while keeping M constant, and thus sliding down an AD curve) why don't we also show them (no questions asked) how to achieve nirvana by doubling M while keeping P constant (thus doubling M/P and shifting the AD curve outwards)?  

This latter science fiction conjecture would expand AD along a horizontal trajectory (an AS curve?) until we reached capacity constraints.  At the "stroke of a pen" (as Ted Heath once said) we could render redundant a large part of macroeconomics and achive full employment with stable (indeed fixed) prices.  

How many angels can dance on the mythical portions of an AD curve?  How many devils were set loose by the Heath-Nixon attempts to fix P (while pushing central banks to expand M)? 

Robert Leeson