  We prove that, if \mu<\lfloor n/2\rfloor, then every rational plane curve of
degree n and class \mu is a limit of parametrizations of the same degree and
class \mu+1. This property was conjectured in D.Cox, T.Sederberg,F.Chen's
paper: "The moving line ideal basis of planar rational curves" (Comput. Aided
Geom. Des. 15 (1998), 803-827), and its validity allows an explicit description
of the variety of parametrizations of degree n and class \mu, for all (n,\mu).
