groupby algorithm does not cover "select distinct" case

Add a sentence to step 2 of the four-step algorithm: Applies set transformations to each subset according to the second parameter, resulting in a new set of potentially different structure and cardinality. If the second parameter is absent, transforms each subset into a set consisting of a single instance without properties, Rephrase the later remark about "select distinct": If the second parameter can be is omitted, steps 2 and 3 produce one instance containing only the grouping properties per to request distinct value combination of the grouping properties.