The "formula" x ( y F(x, y) y M(x, M(y)))
is not a meaningful formula in predicate logic. The "subformula"
M(y) serves as an argument to the predicate M in M(x, M(y)). This is not allowed
as arguments to predicates have to be terms. Inspect the
definition of predicate logic formulas in Chapter 2 for details.
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