abstract class f_xj extends Mathd
{
  // single function  multi independent variable
  // a single value is returned indiced to equation_ref
  // example f[0]=x[0]+sin(x[1])
  //         f[1]=x[0]*x[0]-x[1]
  // func(x,1) returns the value of f[1]
  // func(x,0) returns the value of f[0]

  abstract double func(double x[]);
}
