EISL was written by Kenichi Sasagawa
https://qiita.com/sym_num/items/793adfe118514668e5b0
The license appears to be similar to BSD/MIT to my non-laywer eyes:
https://github.com/sasagawa888/eisl/blob/master/license.txt
It provides an interface to WiringPi, allowing control of the GPIO pins on a Raspberry π.