Surface normal of a triangle

 
function face_normal(point_a,point_b,point_c)=
let(u=point_a-point_b,v=point_a-point_c)
un([u[1]*v[2]-u[2]*v[1],u[2]*v[0]-u[0]*v[2],u[0]*v[1]-u[1]*v[0]]);

function un(v)=v/max(len3(v),1e-64);
function len3(v) =len(v)>1?

sqrt(addl([for(i=[0:len(v)-1])pow(v[i], 2)]))

:len(v)==1?v[0]:v; 

function addl(l,c=0)= len(l)-1 >c?l[c]+addl(l,c+1):l[c];