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Re: Re: All combinations

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On 12 Apr 2004, at 16:44, Harold Noffke wrote:

> Andrzej:
> Where is the form "S_List" documented?  What does this notation, with
> the underscore embedded between two strings, signify?  I don't recall
> seeing it before.
> Thanks.
> Harold
It represents a pattern named S whose head is List. If you define a 
function f using

f[S_List]:= body

then f[something] will return the result of substituting evaluated 
something into body only if evaluated something has head S, otherwise 
the LHS will be returned unevaluated. This has many uses. One is that 
you cna define f to use a different rule for different heads, e.g.

f[S_List]:= body1


will use body1 only for lists and body2 for everything else. Another 
use is to prevent your function form trying to work on input for which 
it does not make sense.


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