This is the "plait" package, which implements the Plait language as
used with
#lang plait
....
The package is meant to be installed with the Racket package manager:
* In recent versions of DrRacket, choose "Install Package.."
from the "File" menu, and enter
plait
in the dialog.
* From the comamnd line:
raco pkg install plait
The Plait language is based on "plai-typed", but made more consistent.
The "plai-typed" language, in turn, is based on the "plai" language,
which originates from the book "Programming Languages: Application and
Interpretation" by Shriram Krishnamurthi.