IsTangentConnected.cs

        private static bool IsTangentConnected( Solid3D.Face face0, Solid3D.Face face1, Solid3D.Edge edge)
        {
            var angle = 1.5 / 180 * Math.PI;

            // Then the two faces share an edge.
            var surfaces0 = face0.Parametric;
            var surfaces1 = face1.Parametric;

            Vector3D Normal(Surface[] surfaces, Point3D p)
            {
                return surfaces
                    .Select
                    (s =>
                   {
                       // The below call to Project is very slow
                       if (!s.Project(p, out var coord2D))
                           throw new Exception("Unexpected");
                       ;
                       return (p: s.PointAt(coord2D), n: s.Normal(coord2D));
                   })
                    .MaxBy(q => p.DistanceTo(q.p))
                    [0].n;
            }

            var pl = edge.Curve.PointAt(edge.Curve.Domain.Low);
            var pm = edge.Curve.PointAt(edge.Curve.Domain.Mid);
            var ph = edge.Curve.PointAt(edge.Curve.Domain.High);

            var vl0 = Normal(surfaces0, pl);
            var vl1 = Normal(surfaces1, pl);

            var vm0 = Normal(surfaces0, pm);
            var vm1 = Normal(surfaces1, pm);

            var vh0 = Normal(surfaces0, ph);
            var vh1 = Normal(surfaces1, ph);

            var r = Vector3D.AngleBetween(vl0, vl1) < angle
                    && Vector3D.AngleBetween(vm0, vm1) < angle
                    && Vector3D.AngleBetween(vh0, vh1) < angle;

            return r;

        }