I'm trying to work through "Elements of Functional Languages" by Martin Henson. On p. 17 he says:
$v$ occurs free in $v$, $(\lambda v.v)v$, $vw$ and $(\lambda w.v)$ but not in $\lambda v.v$ or in $\lambda v.w$. And $v$ occurs bound in $\lambda v.v$ and in $(\lambda v.v)v$ but not in $v$, $vw$ or $(\lambda w.v)$. Note that free and bound variables are not opposites. A variable may occur free and bound in the same expression.
Can someone explain what this means?