Ultrasonic h library

I have been trying to use the "Ultrasonic.h library.
I have imported two different Ultrasonic library sources and come up with the same error message in verify:

In file included from sketch-jan24a.ino:20:0:/home/pi/rf24/libraries/src/ultrasonic.h:
38:5:error:'boolean' does not name a type
boolean three Pins=false;
^
This error message sends me to a library which I can't open. The library should be alright because its been around for years. I am trying out Ultrasonic sensors HC-SR04.
Any thoughts would be appreciated.
TECHBUG

Post a link to where you got the library from. Please use the chain links icon on the toolbar to make it clickable. Or if you installed it using Library Manger(Sketch > Include Library > Manage Libraries) then say so and state the full name of the library.

That error would be caused by the library not including Arduino.h.

The boolean type is a non-standard type alias Arduino created for the standard bool type because apparently they thought boolean was more user friendly. I was able to get the documentation updated to recommend the use of bool instead of boolean and have received approval to modify all official Arduino code to use bool instead of boolean.

TechBug:
I have been trying to use the "Ultrasonic.h library.
I have imported two different Ultrasonic library sources and come up with the same error message in verify:

In file included from sketch-jan24a.ino:20:0:/home/pi/rf24/libraries/src/ultrasonic.h:
38:5:error:'boolean' does not name a type
boolean three Pins=false;
^
This error message sends me to a library which I can't open. The library should be alright because its been around for years. I am trying out Ultrasonic sensors HC-SR04.
Any thoughts would be appreciated.
TECHBUG

the library (link?) may not have

#include "Arduino.h"

where the keyword boolean is defined; it is not a regular C++ keyword (a rather pedantic treatment in the Arduino libraries added for all of those people who could interpret what an int was but couldn't find their way to bool).

try adding the include or just changing the keyword to the proper C++ bool