given a list of facts such as
task(a,5).
task(b,4).
task(c,1).
task(d,7).

How would you define a predicate with a rule that will enable you to find the total time of the tasks. the integers in the facts stand for the time allocated for each task e.g.

lets call this predicate i want to define add:

?- add([a,b,c],TotalTime).
Prolog should resond with

TotalTime=10.