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Warning from Piecewise


When I try to set a delayed expression like this:

Comm[Piecewise[A:{{_, _}..}], Piecewise[B:{{_, _}..}]] :=
   Comm[Testy[A], Testy2[B]]

I get a warning from Piecewise that says:

Piecewise::pairs: The first argument A : {{_, _} ..} of Piecewise is not 
a list of pairs.

I thought that the HoldAll attribute on SetDelayed should prevent 
evaluation of the LHS argument, so where is this warning coming from?

Can I ignore a warning like this, or should I instead use a different 
sort of pattern?

Many thanks,

Chris Rodgers.

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