X-Simple Region

X-Simple Definition

A region $R\subset {\mathbb{R}}^2$ is x-simple if there are continuous functions ${\psi }_1$ and ${\psi }_2:[c,d]\to \mathbb{R}$ s.t: $R=\{(x,y):{\psi }_1(y)\le x\le {\psi }_2(y)\vee c\le y\le d\}$

Intuition

There are curves bounding x