  == Hyperbolic case ==  I can't make sense of the following:  :In hyperbolic geometry, for a right triangle one can also write,  ::  :where  is the angle of parallelism of the line segment AB that  where μ is the multiplicative distance function (see Hilbert's arithmetic of ends).  * The sentence doesn't quite parse.  * Is  really related to the segment AB, rather than to BC? That is weird notation. * Why are we told about  even though a doesn't even show up in the formula? * In the sequel, the letters a, b and c are used. Are they still the ordinary side lengths of the right triangle, as in the rest of the article?   