The statement ‘xs = unknown’ (or ‘xm = unknown’, respectively) is used to indicate that the
variable xs (or xm, respectively) has not been assigned an actual value; xs (or xm) is then
called unknown. The constant unknown has a special meaning: it expresses that the inference
engine has not been able to derive one or more values for the variable. Since its meaning
goes beyond (that is, meta) the contents of the fact set and the knowledge base, the constant
unknown is called a meta-constant.