Constrain slope in segmented package

Hello,

I'm currently using the segmented package of M.R. Muggeo to fit a
two-slope segmented regression. I would like to constrain a
null-left-slope, but I cannot make it. I followed the explanations of
the package (http://dssm.unipa.it/vmuggeo/segmentedRnews.pdf) to write
the following code :

    fit.glm <- glm(y~x)
    fit.seg <- segmented(fit.glm, seg.Z=~x,psi=0.3)
    fit.glm <-update(fit.glm,.~.-x)
    fit.seg1 <-update(fit.seg)

If I got well, the two last lines are supposed to constrain the slope,
but it did not change anything. Could you help me with this ?
By the way, I don't understand the ".-.-x" code in the update function.

Thanks a lot by anticipation !

Pierre