  To Rogers (1994) we owe the insight that monadic second order predicate logic
with multiple successors (MSO) is well suited in many respects as a realistic
formal base for syntactic theorizing. However, the agreeable formal properties
of this logic come at a cost: MSO is equivalent with the class of regular tree
automata/grammars, and, thereby, with the class of context-free languages.
  This paper outlines one approach towards a solution of MSO's expressivity
problem. On the background of an algebraically refined Chomsky hierarchy, which
allows the definition of several classes of languages--in particular, a whole
hierarchy between CF and CS--via regular tree grammars over unambiguously
derivable alphabets of varying complexity plus their respective
yield-functions, it shows that not only some non-context-free string languages
can be captured by context-free means in this way, but that this approach can
be generalized to the corresponding structures. I.e., non-recognizable sets of
structures can--up to homomorphism--be coded context-freely. Since the class of
languages covered--Fischer's (1968} OI family of indexed languages--includes
all attested instances of non-context-freeness in natural language, there
exists an indirect, to be sure, but completely general way to formally describe
the natural languages using a weak framework like MSO.
