Avoid any hint of needing the tangent until after the primal loop has converged.
Avoid any hint of needing the tangent until after the primal loop has
converged.

This allows the dead code eliminator to avoid computing it, at least
in the heron method example.

Also more todos.

axch/dysvunctional-language,axch/dysvunctional-language,axch/dysvunctional-language,axch/dysvunctional-language