Derivation of $ISO(p,q)$ from $SO(p,q+1)$ by contraction? In this preliminaty book it says on page 261 that one can obtain inhomogenous special orthogonal group $ISO(p,q)$ from the special orthogonal group $SO(p,q+1)$ from a so-called contraction which means 
rescaling some generators by a parameters $\epsilon$ and then taking the limit $\epsilon \rightarrow 0$ or $\epsilon \rightarrow \infty$.
I dont understand how this works exactly. What does get contracted in this process? Could somebody spell out some more details, at best by means ot the example at hand?

Contractions are defined for Lie groups, Lie algebras and algebraic groups, and play an important role in in mathematics and physics. For a kind of survey see my article. The easiest example is on the level of Lie algebras, where one could start with the contraction of the $3$-dimensional Heisenberg Lie algebra to the abelian Lie algebra $\mathbb{R}^3$ by $g_{\epsilon}=\epsilon^{-1}{\rm id}$ and $\epsilon \to 0^+$. This reflects the well-known example of the limit process from quantum mechanics to
classical mechanics under $\hbar \rightarrow 0$, which corresponds to the contraction of the
Heisenberg Lie algebra to the Abelian one of the same dimension.
