The x ( y F(x, y) y G(x, y))
contains meaningful subformulas, but it also contains formulas that
don't have meaning, or that are at least underspecified.
- The subformula y F(x, y) means
that 'x is a father'.
- The subformula
y G(x, y)) does not mean
anything since the meaning of the symbol G is not defined,
and since the specifications only told us to make use of F and M.
Thus, you need to express a predicate for "grandmother" in terms of
F and/or M.
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