This modules crops\index{crop!a curve} 
a set of Fcurves with a rectangular box. All connected
components of the intersection of the box with the input curves are
stored in a Fcurves structure (which may be NULL). 
The intersection points at the border
of the box are not computed: only the points of the
input curves lying inside the box are kept.
The \verb+-b+ option produces an additional Fcurve ($box$) that represents 
the croping box given as an input to the module.
