i find this :
http://www.mathworks.com/access/helpdesk/help/toolbox/mupad/plot/PRIMITIV_Implicit2d.html
but i try the example:
plot(plot::Implicit2d(x^2+y^2-1, x = -1..1, y = -1..1))
and matlab find an error..
>> plot(plot::Implicit2d(x^2+y^2-1, x = -1..1, y = -1..1))
...