Is the (?=)/2 condition remanent of the andorra model? http://ciao-lang.org/docs/ciao/andorra_doc.html
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:- use_module(library(when)).(can be autoloaded)
. See also freeze/2 and dif/2. The implementation can deal with cyclic terms in X and Y.
predicate is realised using attributed variables associated with the
when. It is defined in the autoload library