“…Although the principal symbols are calculated in the interior in [Guillemin and Melrose 1979b, Proposition 5.1;Marvizi and Melrose 1982, Section 6;Petkov and Stoyanov 1992, Section 6], the results do not seem to be stated along the boundary (i.e., the symbols are not calculated at the boundary). The statements we need are contained in Theorem 3.1 of [Chazarain 1976] (and its proof), and we largely follow its presentation. We need to calculate the canonical relation and principal symbol of the wave group, its derivatives and their restrictions to the boundary.…”