JESS: MIME-Version: 1.0

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I tried the following JESS code from JIA on page 101,

Jess> (assert (letters b c))
<Fact-0>
Jess> (defrule literal-values
(letters (__data b c))              <<----  *
=>)


when i entered the second line of the defrule code (marked * above) and pressed 'enter' to write the next line, I got this error message:

Jess reported an error in routine Jesp.parsePattern.
  Message: An ordered template can't have named slots at token '('.
  Program text: ( defrule literal-values ( letters (  at line 139.
Jess> __data
Jess> b
Jess> c
Jess> Jess reported an error in routine Jesp.parseExpression.
  Message: Expected a '(', constant, or global variable at token ')'.
  Program text: )  at line 139.
Jess> Jess reported an error in routine Jesp.parseExpression.
  Message: Expected a '(', constant, or global variable at token ')'.
  Program text: )  at line 139.

I'm using Jess Version  7.0b6  (3/13/2006)

why is this happened?

thanks