Goal:
  
  elt : Type
  bst_dec : forall t : M.Raw.tree elt, {M.Raw.bst t} + {~ M.Raw.bst t}
  t : M.Raw.tree elt
  ============================
   (M.Raw.bst (M.Raw.Leaf elt))


DEBUG:Going to execute:
solve [ constructor; assumption | inversion 1; subst; auto ]