This function declares an optional type, a union of the named class (which
must already be defined) and NULL. It is meant for use in packages.
Optional(className)The name of the union class (invisibly), which will be the original class name with "Optional" prepended to it.
A string naming an existing (S4) class.
Jon Clayden
Please cite the following reference when using TractoR in your work:
J.D. Clayden, S. Muñoz Maniega, A.J. Storkey, M.D. King, M.E. Bastin & C.A. Clark (2011). TractoR: Magnetic resonance imaging and tractography with R. Journal of Statistical Software 44(8):1-18. tools:::Rd_expr_doi("10.18637/jss.v044.i08").