I read in a paper a definition like this:
Given $\Sigma=\{\neq,=, >,<,\leq,\geq \}$, we define a constraint $v\ op \ w$ with $v,w \in \mathbb{R}$ and $op \in \Sigma$. Moreover, we indicate by $\neg (v\ op \ w)$ the constraint in which $op$ is replaced by its negation.
(I have synthesized the definition)
Now, the question is: is well-defined to say a negation of a relational operator?