（序言）获取没有列表的变量总和

Question

very new to Prolog and I'm absolutely awful at it.

I am trying to get a sum for all the marks received on assignments, and return it. The sum predicate must be of the form sumAssignments(S) .

Here's the knowledge base I've made:

assignment(1, A).
assignment(2, B).
assignment(3, C).
assignment(4, D).
assignment(5, E).

...Where assignment(1, A) means that assignment 1 has a variable grade A (could be 70, could be 50, etc.).

Here is my attempt at getting the sum, just for the first two assignments for testing purposes:

sumAssignments(S) :- assignment(1, A), assignment(2, B), S=A+B.

That always returns yes. The key here is I can't use lists.

Answer 1

I found out that only constants belong in predicates, so instead of assignment(1, A). we would find an arbitrary number to represent 'A', in this case maybe 67 or 100: assignment(1, 67). The summation works fine after doing that.