![ClearAll[f] Attributes[f] = {Flat} ; f[x_, y_] := {x, y} Attributes[f] = {} ... [1, 2, 3] (* The head f is Flat for pattern matching . *)](../HTMLFiles/Tricks_A-K_435.gif)
When the next input cell is evaluated (f) is Flat for pattern matching, but not for evaluation.
![{f[1], {f[2], f[3]}}](../HTMLFiles/Tricks_A-K_436.gif){kind=small}
![f[f[f[1]]] (* Nested layers of f are not Flattened . *)](../HTMLFiles/Tricks_A-K_437.gif){kind=small}
![f[f[f[1]]]](../HTMLFiles/Tricks_A-K_438.gif){kind=small}
It now seems like (f) has the same definition twice. We can evaluate ( f[x_,y_]=. ) once to get rid of one definition, but we need to use ( f[x_,y_]=. ) again to get rid of the second definition. Alternatively we could use Clear[f] and all definitions for (f) would be removed in one step.
Global`f
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![f[x_, y_] =.](../HTMLFiles/Tricks_A-K_440.gif){kind=small}
Global`f
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![f[x_, y_] =.](../HTMLFiles/Tricks_A-K_442.gif){kind=small}
Now (f) has no definitions.
Global`f
![{f[1, 2], f[f[1]]}](../HTMLFiles/Tricks_A-K_444.gif){kind=small}
![{f[1, 2], f[f[1]]}](../HTMLFiles/Tricks_A-K_445.gif){kind=small}
Created by Mathematica (May 16, 2004)