Play with two points on two spheres

Guess conditions that make the scaling constraint impossible to satisfy.

engine-proto/Engine.Algebraic.jl

@@ -184,6 +184,8 @@ function realize(ctx::Construction{T}) where T
   )
   
   # add relations to center, orient, and scale the construction
+  # [to do] the scaling constraint, as written, can be impossible to satisfy
+  #         when all of the spheres have to go through the origin
   if !isempty(ctx.points)
     append!(eqns, [sum(pt.coords[k] for pt in ctx.points) for k in 1:3])
   end