Example one:
```
spack install --add x y z
```
is equivalent to
```
spack add x y z
spack concretize
spack install --only-concrete
```
where `--only-concrete` installs without modifying spack.yaml/spack.lock
Example two:
```
spack install
```
concretizes current spack.yaml if outdated and installs all specs.
Example three:
```
spack install x y z
```
concretizes current spack.yaml if outdated and installs *only* concrete
specs in the environment that match abstract specs `x`, `y`, or `z`.
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