hello,

I think I hit a bug in next-p. As far as I understand, the following code
should be perfectly legal:

;	p2	p3	p4	p5
i1	0	1	60	np4
i1	1	1	64	60

but when compiling, I get these messages:

swrite: output, sect1 line2 p5 makes illegal reference to n p4 Zero
substituted 
swrite: output, sect1 line3 p6 makes illegal reference to p4 Zero
substituted

Two things are wrong:

- in the first event it claims that the reference to np4 is illegal
- in the following event, it sees an inexistent p6

Please see attached a complete example csd.

I'm using:
--Csound version 6.12 beta (double samples) May 31 2018
[commit: bef59a7a9d4de7b211fe9e236f35b21cbd9948a9]

I think it was working in 6.10 or something. 



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