[Nix-dev] How to install specific version of a package?

[Nick Sabalausky]

In general, how do you install a specific version of a package? It seems 
that by default it will install the latest available version like other 
distros, but for the most part installing "packagename-1.2.3"/etc 
doesn't appear to work.

One of the biggest killer features of NixOS for me is the ability to 
sanely handle and manage different versions of the same package in 
different environments. But that ability seems severely crippled if I 
can't actually *install* any versions other than "latest" and "already 
previously installed on this machine while it *was* the latest, and 
hasn't yet been garbage collected".