[...] α is X-free if and only if is not X-bound.
- Chomsky (1993), a pag.185

[...] α is X-free if and only if there is no β, γ such that (i) β X-binds α; (ii) if γ Ᾱ-binds α, then γ = β or γ binds β.
- Chomsky (1993), a pag.201