[m-dev.] for review: line numbers for output streams

[Thomas Charles CONWAY]

> This change was requested by Peter Szeredi and is also needed for
> us to output proper #line directives in the generated C code.
> 
> Tom, can you please review this one?
> 
> library/io.m:
> 	Add predicates io__output_line_number/{3,4}
> 	and io__set_output_line_number/{3,4}.
> 	Previously we only supported line numbers for input streams.
> 	Most of the functionality for this was already implemented --
> 	we already computed the line numbers for output streams, there
> 	was just no way to access them.
> 

This change looks fine.

What is the status (I can't remember) of #line directives? Zoltan
had something almost going a while back...

-- 
Thomas Conway               				      conway at cs.mu.oz.au
AD DEUM ET VINUM	  			      Every sword has two edges.