Description: Distribution of class substitution over a left-nested implication.
Similar to sbcimg . sbcim2g is sbcim2gVD without virtual deductions
and was automatically derived from sbcim2gVD using the tools program
translate..without..overwriting.cmd and Metamath's minimize command.
(Contributed by Alan Sare, 18-Mar-2012)(Proof modification is discouraged.)(New usage is discouraged.)