Rich - Here are some experiments with the problem we were discussing.
Suppose we have the following facts:

     
     => full_name(finin,'Tim Finin').
     
     => host_name(finin,antares).
     
     => full_name(robin,'Robin,McEntire').
     
     => host_name(fritzson,hamlet).
     

I first tried representing the user rule and the defaults as follows:

     full_name(User,Name),
     host_name(User,Host) 
       =>
       user(User,Name,Host).
     
     full_name(User,unknown) 
       <= 
       ~full_name(User,X)/(X\==unknown).
     
     host_name(User,unknown) 
       <= 
       ~host_name(User,X)/(X\==unknown).
     
When I add the rules and facts, I get:

     Adding (u) full_name(finin,Tim Finin)
     Adding host_name(finin,unknown)
     Adding user(finin,Tim Finin,unknown)
     Adding (u) host_name(finin,antares)
     Adding user(finin,Tim Finin,antares)
     Removing host_name(finin,unknown).
     Removing user(finin,Tim Finin,unknown).
     Adding (u) full_name(robin,Robin,McEntire)
     Adding host_name(robin,unknown)
     Adding user(robin,Robin,McEntire,unknown)
     Adding (u) host_name(fritzson,hamlet)
     
In other words, every thing is ok except that I haven't deduced a user
relation for fritzson, since I don't have the required full_name
assertion to start the ball rolling.  Now, if I add the following
backward chaining rule:

    user(User,Name,Host)
       <=
      full_name(User,Name),
      host_name(User,Host).

I can now deduce the user relation for fritzson and any others, in fact:

     | ?- pfc(user(fritzson,FN,HN)).
     
     Adding full_name(fritzson,unknown)
     Adding user(fritzson,unknown,hamlet)
     FN = unknown,
     HN = hamlet 
     
     | ?- pfcWhy(user(fritzson,unknown,hamlet)).
     
     Justifications for user(fritzson,unknown,hamlet):
     
         1.1 host_name(fritzson,hamlet)
         1.2 full_name(fritzson,unknown)
         1.3 full_name(A,B),host_name(A,C)=>user(A,B,C)
     
         2.1 host_name(fritzson,hamlet)
         2.2 full_name(fritzson,unknown)
         2.3 user(A,B,C)<=full_name(A,B),host_name(A,C)
      >> q.
     
     yes
     | ?- pfc(user(mcKay,FN,HN)).
     
     Adding full_name(mcKay,unknown)
     Adding host_name(mcKay,unknown)
     Adding user(mcKay,unknown,unknown)
     FN = HN = unknown 
     
     | ?- pfcWhy(user(mcKay,unknown,unknown)).
     
     Justifications for user(mcKay,unknown,unknown):
     
         1.1 host_name(mcKay,unknown)
         1.2 full_name(mcKay,unknown)
         1.3 full_name(A,B),host_name(A,C)=>user(A,B,C)
     
         2.1 host_name(mcKay,unknown)
         2.2 full_name(mcKay,unknown)
         2.3 user(A,B,C)<=full_name(A,B),host_name(A,C)
      >> 1.1 .
     
     Justifications for host_name(mcKay,unknown):
     
         1.1 ~host_name(mcKay,A)
         1.2 host_name(A,unknown)<= ~host_name(A,B)/ (B\==unknown)
      >> q.
     
     yes
     | ?-