> The Spider Objects

>       Pre-Image by MDS given KPCA coordinates

>        a = pmg_fp(alg,hyper)
>        generates a pre-image object; uses the fix-point method
>        -------------------------------------------------------
>        hyperparameters:
>        child=kernel     kernel to work with.
>        map='rbf_inv'	   function to convert kernel distance back to input distance
>        eps=1e-5         epsilon
>        n=5         	   number of nearest neigbhors
>        kp=kpca		   kpca object
>        dn=sample data   data pool, if empty uses kp.dat
>        stats:
>        methods:
>        train         calls test
>        test			find the preimage

>        example:
>        k=kernel('rbf',1.5);

>        d=gen(toy2d);
>        d2=gen(toy2d);
>        [r,kp]=train(kpca({k,'feat=20'}),d2);
>        p0=pmg_mds;
>        p0.kp=kp;
>        p0.child=k;
>        p0.n=10;

>        r=test(kp,d);

>        reconstruct=test(p0,r)

>        clf;
>        hold on;
>        plot(d2.X(:,1),d2.X(:,2),'g.');
>        plot(d.X(:,1),d.X(:,2),'r.');
>        plot(reconstruct.X(:,1),reconstruct.X(:,2),'o');

> | Reference : The Pre-Image Problem in Kernel Methods

> | Author : James Kwok et al.