Problem #2: "Syntax error: Operator priority clash"

Description

Solution

Replace the comma on the last line with a period.

SWI-Prolog uses the term for illegal sequences of operators. I.e.
a :- b :- c. 

Quoted from: http://www.groupsrv.com/computers/about124076.html 

Comment

Even though I new to Prolog (this is my second program using it), I feel like I'll continue to make this mistake if I ever use Prolog again.