Massachusetts Institute of Technology: Srini Devadas and Eric Lehman's "Counting I"
Read through Section 1.4. This work presents some strategies and rules that aid us in counting members of sets arising in the analysis of probability problems.
Section 1.3 illustrates the bijection rule for arrays and lists. The bijection rule can be applied to counting the elements of an array. The elements of a list can be mapped via a bijection to a one-dimensional array. Thus, because we can count the elements of a list, we can count the elements of such an array. (We count the elements of a list, by walking down the list and incrementing a tally, initialized to zero, by one for each item of the list.)
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