Os consist of a symbol followed by a sequence of zero or more fields separated by spaces and delimited by an opening parenthesis on the left and a closing parenthesis on the right. The first field of an ordered fact specifies a ìrelationî that applied to the remaining fields in the ordered fact. For example, (fatherof jack bill) states that bill is the father of jack.
Some examples of ordered facts are shown following.
(the pump is on) (altitude is 10000 feet) (grocery-list bread milk eggs)
Fields in a nonordered fact may be of any of the primitive data
types (with the exception of the first field which must be a symbol),
and no restriction is placed on the ordering of fields. The following
symbols are reserved and should
not be used as the first field in any fact (ordered or
nonordered): , ,
, , ,
, , ,
and . These words are reserved only
when used as a deftemplate name (whether explicitly defined or
implied). These symbols may be used as slot names, however, this
is not recommended.