A combinatorial proof of a symmetric -Pfaff-Saalschütz identity.
We present a short and self-contained proof of the extension property for partial isometries of the class of all finite metric spaces.
Research on combinatorial properties of sequences in groups and semigroups originates from Bernhard Neumann's theorem answering a question of Paul Erd"{o}s. For results on related combinatorial properties of sequences in semigroups we refer to the book [3]. In 2000 the authors introduced a new combinatorial property and described all groups satisfying it. The present paper extends this result to all semigroups.