load(draw)$ R:15; r:5; eps:0.05; x:(R + r * cos(phi)) * cos(theta); y:(R + r * cos(phi)) * sin(theta); z:r * sin(phi); draw3d( terminal      = svg,         file_name     = "Torus2",         xtics         = none,         ytics         = none,         ztics         = none,         axis_3d       = false,         view          = [90,330],         xu_grid       = 50,         yv_grid       = 100,         surface_hide  = true,         color         = black,         enhanced3d    = false,         wired_surface = true,         color         = gray,         line_width    = 0.5,         parametric_surface(z,y,x,phi,-%pi,%pi,theta,-%pi,%pi),         color         = red,         line_width    = 1,         parametric((r+eps)*sin(t), 0, R + (r+eps)*cos(t), t,-%pi,%pi),         color         = magenta,         parametric(-r, R*sin(t), R*cos(t), t,-%pi, %pi),         xrange=[-20,20]        );