Sentence:  If one has a <WSD>bag</WSD> of three apples and the letter 'a' is taken to denote 'apple', then such <WSD>bag</WSD> could be represented symbolically as {a,a,a}. Note that in an ordinary context, when talking about a <WSD>bag</WSD> of apples, one does not care about identifying the individual apples, although one might be interested in distinguishing apples by species, for example, letting 'r' denote 'red apple' and 'g' denote 'green apple', then a <WSD>bag</WSD> of three red apples and two green apples could be denoted as {r,r,r,g,g}. 
Meaning of the word marked with <WSD> tags:  A collection of objects, disregarding order, but (unlike a set) in which elements may be repeated.