1. The inference

where $\to$ is a conditional connective, or the corresponding axiom scheme

• • $\phi \to \psi \to \left(\xi \to \phi \right)\to \left(\xi \to \psi \right)$

Prefixing is frequently encountered in relevant logic, as is its dual, suffixing.

2. Describes any deductive system $\text{L}$ in which the rule form of prefixing is an admissible inference.