There is a specific minimal core of Lisp, which consists of a certain minimal set of data structures, operations and an evaluation mechanism.
An early version is described here:
https://dspace.mit.edu/bitstream/handle/1721.1/6096/AIM-008....
Another one is described here:
Cons/car/cdr are far less important, IMO, than eval.
Read, on the other hand, is also technically missing from Janet, and I personally don't mind a great deal, but I could see that being argued.
That's part of the core of the language.
Sure: Many other aspects from Lisp can be found also in Lisp-derived languages and other languages. Like all or some of runtime code loading, symbolic expressions, using symbolic expressions to encode source code, saving and starting heap images, garbage collection, managed memory, source level interpreters/debuggers, self-hosted compilers, read-eval-print-loops, macros, fexprs, integrated interactive development environments, ...
Even McCarthy's lisp had iterations of itself.
Personally, I think the second paper actually points to the essential component for me:
> LISP data are symbolic expressions that can be either atoms or lists. Atoms are strings of letters and digits and other characters not otherwise used in LISP. A list consists of a left parenthesis followed by zero or more atoms or lists separated by spaces and ending with a right parenthesis. The LISP programming language is defined by rules whereby certain LISP expressions have other LISP expressions as values.
I would use that to generalize to a family of languages like so:
""" A Lisp language consists of:
1) Some data representation which are symbolic expressions that can be either atoms or lists. Atoms are strings of letters and digits and other characters not otherwise used in the given Lisp language. A list consists of a starting character (commonly left parentheses) followed by zero or more atoms or lists separated by seperator characters (commonly spaces) and ending with a terminating character (commonly right parenthesis).
2) A programming language whose code uses that data representation and is defined by rules whereby certain Lisp expressions have other Lisp expressions as values. """
Personally I think that's the key aspect, the characteristics of what those rules are, and what the chosen characters are is what creates the family of language, they are the parameters you're allowed to change and must change for it to be a dialect and not just an alternate implementation of the same language.
Not even that. It's the programmatic definition of the core of Lisp. McCarthy's real Lisp implementation was much more featureful.
I would also think the Lisp program in the second paper is 'Lisp Program Zero'. ;-) If your language can run it, it is Lisp. The amount of modifications necessary (syntactic, operators, semantics, ...) gives an indication how far another language is away from it.
Here is a version in Common Lisp:
https://gist.github.com/lispm/d752d5761f7078de4041d4e453e70c...