Definition:Well-Formed Part

Let $$\mathbf{A}$$ and $$\mathbf{B}$$ be WFFs of predicate calculus.

Then $$\mathbf{B}$$ is a well-formed part of $$\mathbf{A}$$ iff;
 * $$\mathbf{A} = S \mathbf{B} T$$

for some strings $$S$$ and $$T$$.