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"UnFullForm"ing an Expression?

• To: mathgroup at smc.vnet.net
• Subject: [mg13151] "UnFullForm"ing an Expression?
• From: siegman at ee.stanford.edu (AES)
• Date: Mon, 13 Jul 1998 07:42:03 -0400
• Organization: Stanford University
• Sender: owner-wri-mathgroup at wolfram.com

I recently posted a query noting that if you enter the following

   f1[a_,x_] :=a Cos[x] + a^2 Sin[x]

   f2[a_,x_] := D[f1[a,x],x]

you can make a Table[ ] of f1[a,x] but not of f2[a,x].  

If I look at the FullForms for these, e.g.

   f1[a,x] //  FullForm

   f2[a,x] // FullForm

they both look like simple functions to me (i.e., f2[a,x] shows no
visible memory of having originated from a derivative), except the f2
is inside a "FullForm[ ]" wrapper and f1 isn't.

Is there a way to "UnFillForm" f2 ? If so, would f2 from then on act
like f1 ?  Does this query make any sense?

I'd like to understand the situation here.

Thanks   siegman at ee.stanford.edu