Proposed manual change

[Carl Sorensen]

I would like to propose moving one section of the Extending manual to the
Notation Reference.

The criterion we have used before for deciding what needed to go in the
Extending manual was that if it needed Scheme code, it should be in
Extending.  I see some rationale behind this, but I'd like to propose an
exception.

In working with overrides, we need to introduce some Scheme data structures
(such as lists used for property overrides).  I don't think that's
inappropriate.

The simplest form of music function, the simple substitution function,
requires virtually no knowledge of scheme, as all of the output is created
in .ly code.  Granted, there is a Scheme wrapper, but the Scheme wrapper is
a constant.  All that is needed is to decide upon the variable types, figure
out the type checks, and put the .ly code in the appropriate place.   I
believe this level of sophistication is available to the advance LilyPond
user who is not a Scheme programmer.  I propose that this section go into
the NR.

I've made a patch set to show what I'm thinking.  Please review it:

http://codereview.appspot.com/179098

Thanks,

Carl