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MathGroup Archive 2005

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Re: To be or not to be...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg62039] Re: [mg62002] To be or not to be...
  • From: Kristjan Kannike <kkannike at physic.ut.ee>
  • Date: Wed, 9 Nov 2005 05:06:03 -0500 (EST)
  • References: <200511090845.DAA17323@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Wed, 9 Nov 2005, Ingolf Dahl wrote:

> Try the following commands in order:
>
> b = Sequence[]
>
> b == Sequence[]
>
> b != Sequence[]
>
> b === Sequence[]
>
> b =!= Sequence[]
>
> The four last lines all evaluate to "True" for me. Quite amazing!

There is really nothing mysterious. The FullForm of e.g.

b==Sequence[]

would be

Equal[b, Sequence[]].

As an empty sequence is spliced into the argument list of Equal, the
expression is equivalent to Equal[b] that is _defined_ to be True (check
the Help). Similarly for != or UnEqual. Try

a==Sequence[]

for an undefined a, and it evaluates to True as well.

Kristjan Kannike
<http://www.physic.ut.ee/~kkannike/>


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