notes

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This example produces primary reflections from interfaces 1 and 4.

Since we don't want primaries from all interface we say n 
in Record 17.  Following Record 17 we can describe as many reflection
events as we like by specifying the reflecting interfaces.  Here
there are two events, a reflection from interface 1 and another from
interface 4.

We could also specify multiples after Record 17.  Try for example, an event like
4 2 4
This is a reflection from 4, followed by a reflection from 2,
followed by a reflection from 4 (and thence back to the upper surface).
As you can see, multiples are now specified by reflecting interfaces only.
This is a change from the old code where a complete list of intersections
had to be given.  Note, however, that it is possible to specify an invalid
event now, eg
4 2
After the downward reflection from 2 the rays never come back to the
upper surface.  For a valid event, the rays must end up at the receivers.