A runtime is how we choose to implement a language once a program has been compiled and is now running.
For example, in Arc 3.2, `cons` creates an immutable Racket pair, `car` works with a Racket pair or a Racket symbol nil, and `scar` modifies an Arc cons with Racket's `unsafe-set-mcar!`.
These are all runtime decisions. We could create a different runtime. For example, Arc's `cons` could create a Racket mpair, and `scar` could use Racket's `set-mcar!`.
For Amacx I've written two runtimes so far. One I called "mpair" because it implements Arc lists using Racket's mpairs. The other I called "srcloc" because it allows source location information to be attached to Arc forms (lists and atoms).
Currently, in srcloc, source location information is attached to forms using Racket's syntax objects. Thus, in srcloc, Arc's `car` can be applied to a Racket syntax object which wraps a list or nil, and it will unwrap the syntax object and return the underlying value.
It's a coincidence that I chose to call my runtime "srcloc" and Racket happens to also store source location information in a struct they call "srcloc" :-)