Clusters of s with plosives

Words like 'spill', 'still', 'skill' are usually represented with the phonemes p, t, k following the s. But, as many writers have pointed out, it would be quite reasonable to transcribe them with b, d, g instead. For example, b, d, g are unaspirated while p, t, k in syllable-initial position are usually aspirated. However, in sp, st, sk we find an unaspirated plosive, and there could be an argument for transcribing them as sb, sd, sg. We do not do this, perhaps because of the spelling, but it is important to remember that the contrasts between p and b, between t and d and between k and g are neutralized in this context.