Test of working with strings.
The predicates  'length','head','tail' and 'addString'
must be defined as builtins.

s("abc").

S(V),length(S,N),greater(N,"1"),head(S,H),tail(S,T),inverse(T,It),addString(Ti,H) :> inverse (S,Si).
S(V),length(S,"1") :> inverse (S,S).
S(V),length(S,"0") :> inverse (S,S).

Query: inverse(s,S).