"guarded" fetch from hashed array

• Subject: "guarded" fetch from hashed array
• From: Richard Fateman
• Date: Tue, 07 Feb 2012 13:34:58 -0800

Here is how you make all non-entries produce an error.
hash[x]:= error("ERROR ", x);

/* try it out */

hash[a]:aaa;



hash[a];

  hash[b];

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