Domain R.C. Join Example
"Find employees of the Research dept. whose pay is over $25,000"
(? p) (? d1) (EMPLOYEE (fn,m,ln,s,b,a,x,p,u,d1)
(? d2) (? dn) DEPARTMENT (dn,d2,mg,ms)
and d1= d2 and dn="Research"}
Many of the variables are just placeholders.
In Prolog you could use "anonymous variables" (_).