As I can read in the "Clips_basic_programming_guide.pdf" the SYMBOL data type is defined like this:
"A symbol in CLIPS is any sequence of characters that starts with any printable ASCII character and is followed by zero or more printable ASCII characters." (page 7)
However I can not create this kind of instances:
(defclass ORDER (is-a USER)
(slot state (allowed-symbols 1 2 3 4 5 a b c d))
(deftemplate order (slot state (allowed-symbols 1 2 3 4 5 a b c d)))
I think the problem is that the values 1, 2, 3, 4 and 5 are INTEGER types but they should be also SYMBOLYC, don't?
CLIPS> (integerp 2)
CLIPS> (symbolp 2)
FALSE -> (Shouldn't be TRUE?)
If you read a little further you'll see the following:
Note that numbers are a special case of symbols (i.e. they satisfy the definition of a symbol, but they are treated as a different data type).