Backtrack before dispatch, [do_regularize] does everything again.

Revert "Type annotations result in a clearer error message from Program."
This reverts commit 766b48cbcc5d687d98f4978c5663d9be459d2c31.

Revert "[do_regularize] now dispatches to dedicated function (Program still fails)."
This reverts commit 4e46fb378cfc21eec684509c3e1b3b296eca64e1.
Program problem located, handled, everything type checks.

But there still is [Parameter discrim] which is not implemented.