UWN

There is more, (also a bit in Scryer), will tell you when Scryer is ready.

Here are [all tests](https://www.complang.tuwien.ac.at/ulrich/iso-prolog/phrase). Consider to include `phrase//2` and `phrase//3` as higher-order extensions.

Shorter: ``` ?- _==_. true, unexpected. false. % expected

Yet, ``` ?- _=1,_=2. true. % as expected

``` ?- copy_term(_+_,T). T = __1+__2. % expected, perfect! ```

``` ?- sort([_,_],L). L = [_], unexpected.

``` ?- X=_,Y=_,L=X-X. X = Y, _ = Y, L = Y-Y, unexpected. ``` (All examples, I enter at [the first page](http://tau-prolog.org/) directly. So the prompt there is actually ">"...

@jfmc: are you aware of another system that fails here? At least SICStus, Scryer, Trealla, GNU, Ichiban, ECLiPSe, SWI, Tau, YAP, XSB, B, Minerva, IF, IV all produce an instantiation...

You are suggesting that this is an ISO only feature. But many pre-ISO systems produced this instantiation error as well. Like [Quintus](https://quintus.sics.se/isl/quintus/html/quintus/mpg-ref-functor.html#mpg-ref-functor). And to quote the User's Guide to DECsystem-10...

For the record. A SICStus that already produces the instantiation error: ``` In SICStus 2.1 #5: Sat May 16 22:12:10 MET DST 1992: | ?- [user]. | A. {INSTANTIATION ERROR:...