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Replacement problem


These expressions are condensation of larger ones
(about 700 lines or so each) but they illustrate random
substitution failures in 4.2.  Question: how can the
substitution Sqrt[...]->Q always be made to work?  
The help file under ReplaceAll, ReplaceRepeated, etc, 
does not address this problem.

Thanks


f=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]/4*F^2);
Print[(f/.Sqrt[X+Y+Z]->Q)//InputForm]; 

B*(A + Q) + (4*C)/(F^2*Sqrt[X + Y + Z])  (* fails *)

g=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]*4*F^2);
Print[(g/.Sqrt[X+Y+Z]->Q)//InputForm];

B*(A + Q) + C/(4*F^2*Sqrt[X + Y + Z])    (* fails *)

h=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]+4*F^2);
Print[(h/.Sqrt[X+Y+Z]->Q)//InputForm];

B*(A + Q) + C/(4*F^2 + Q)                (* works *)


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