In order to provide compatibility with Edinburgh Prolog, the KnowledgeWorks backward chaining system treats Prolog structured terms differently from lists. Structured terms whose functors are not `.' are stored as simple vectors with the functor as element 0 (for example, the term: foo(bar) is equivalent to #(foo bar)).
KnowledgeWorks and Prolog User Guide (Unix version) - 24 Mar 2017