+ (id ,string? "An id string for the grob. Depending on the typestting
+backend being used, this id will be assigned to a group containing all of
+the stencils that comprise a given grob. For example, in the svg backend,
+the string will be assigned to the @code{id} attribute of a group (<g>)
+that encloses the stencils that comprise the grob. In the Postscript
+backend, as there is no way to group items, the setting of the id property
+will have no effect.")