 Z_1
| onto a submanifold of X.  We say that f is an &quot;immersion&quot; if it is
| an immersion at every point.
|
| Lang, DARM, page 24.
|
| Note that there exist injective immersions
| which are not isomorphisms onto submanifolds,
| as given by the following example:
|      ________
|     /        \
|    /          \
|   |           |
|   |           |
|    \          V
|     \___________________________________________
|
| (The arrow means that the line approaches itself without touching.)
| An immersi