Purity advice - long wrapped texts

[Mark Knoop]

Hi list,

I'm looking for some advice from scheme experts with regard to writing pure 
functions. I've written a Grob and engraver (attached) for the purpose of 
setting long text instructions which should be wrapped over a section of music. 
Obviously the height of the resulting stencil will vary depending on the 
ultimate width of the spanner.

Thus I've added an unpure-pure container for the Y-extent callback. This is 
basically working, but any advice on how to improve the pure estimate would be 
greatly appreciated.

For now I'm simply using the difference of the paper-column ranks to give an 
indication of the horizontal length. This tends to estimate a larger height 
than is required (which is better than too small), but leads to some wasted 
vertical space.

Thanks,

Mark

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Mark Knoop