The core language has indeed been around for several decades. However, many important features that are necessary for more interesting Prolog programs have beco

The core language has indeed been around for several decades. However, many important features that are necessary for more interesting Prolog programs have become widely available and implemented with acceptable efficiency only in the much more recent past. For example, this concrete Joy interpreter depends on features (call_residue_vars/2, copy_term/3 etc.) that were added to a development version of SWI-Prolog only a few years ago. It takes at least several months, often even years, until development features make it into stable versions and distributions, and until application programmers get accustomed to them or even start using them.