The empty set approach compares two sounds that are unknown to the child (not used - e.g. /l/ vs /tʃ/) in non-homonymous contrasts. If the child’s phonetic inventory consists of: [m, n, t, d, p, b, w, ...
To round out our phonetic journey, here are a few consonant sounds you won’t have to second-guess. One sound you can be certain of is ‘Z’ (as long as it isn’t combined into a digraph or trigraph).