Solid angle at a point spanned by vectors to given points
SolidAngle[p, {u1, …, u d}]
SolidAngle[p, reg]
SolidAngle[{0, 0}, {{1, 0}, {1, 1}}]
→ Pi/4SolidAngle[{0, 0, 0}, {{1, 0, 1}, {1, 1, 1}, {0, 0, 1}}]
→ 2*ArcTan[(2 + Sqrt[2] + Sqrt[3] + Sqrt[6])^(-1)]