'Evaluate the integral of the spline over area [xa,xb] x [ya,yb].
Parameters
xa, xb : float
The end-points of the x integration interval.
ya, yb : float
The end-points of the y integration interval.
Returns
integ : float
The value of the resulting integral.'

def integral(self, xa, xb, ya, yb):

    (tx, ty, c) = self.tck[:3]
    (kx, ky) = self.degrees
    return dfitpack.dblint(tx, ty, c, kx, ky, xa, xb, ya, yb)
