10. The function $f(x)$ defined on $\mathbf{R}$ satisfies
$$
f\left(f(x)-x^{2}+x\right)=f(x)-x^{2}+x \text {. }
$$

If there is exactly one real number $x_{0}$ such that $f\left(x_{0}\right)=$ $x_{0}$, then $f(x)=$ $\qquad$