Definition:Well-Formed Part

Definition
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 $$T$$.