doc-src/Tutorial/Datatype/tconstssubst

fixed subgoal_tac; fails on non-existent subgoal;

consts
   subst  :: ('a => ('a,'b)term) => ('a,'b)term      => ('a,'b)term
   substs :: ('a => ('a,'b)term) => ('a,'b)term list => ('a,'b)term list