Install fltk1.1-doc by entering the following commands in the terminal:
sudo apt update sudo apt install fltk1.1-doc
Description:
Fast Light Toolkit - documentation
This package provides documentation for FLTK, including a chapter on its user interface designer (fluid). . The Fast Light Toolkit (FLTK) is a cross-platform graphical user interface toolkit originally based on libForms.
Homepage: http://www.fltk.org/
Version: 1.1.10-23
Section: universe/doc