We have to repeat all the spec attributes in a number of places in
`concretize.lp`, and Spack has a fair number of spec attributes. If we
instead add some rules up front that establish equivalencies like this:
```
node(Package) :- attr("node", Package).
attr("node", Package) :- node(Package).
version(Package, Version) :- attr("version", Package, Version).
attr("version", Package, Version) :- version(Package, Version).
```
We can rewrite most of the repetitive conditions with `attr` and repeat
only for each arity (there are only 3 arities for spec attributes so far)
as opposed to each spec attribute. This makes the logic easier to read
and the rules easier to follow.
Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>
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