How do I read/work with PolyLineZM with maptools/sp?

Hi all!

I feel like I?m missing something simple. I can access X & Y coordinates like:

s <- readShapeLines(elev)
head(coordinates(s at lines[[1]]@Lines[[1]]))

I can?t seems to find any information on how to access Z and M values. I can easily access those with `shapefiles` package, but it has a limited support for Point Z when writing a shapefile and it is generally not as structured as `maptools`/`sp`. While having Z value as an attribute for an output point is not an issue for me, I do have PolylineZM as an input and I do need an access to both Z & M.

Thank you for any suggestions,
Mikhail