bTree: Correction of a bug in the function 'StringPrefP', which happened with runes of several bytes.
graphQL: Possibility of errors with several positions in text, in accordance with specifications of October 2021; ErrorElem.Location -> ErrorElem.Locations.
sets3: New function 'ElemOfRank', which returns the requested set element at any position in lexicographic order.
strings: New package centralising some current string utilities.