'Parameters
annihilator:
Annihilator of the Holonomic Function, represented by a
`DifferentialOperator` object.
x:
Variable of the function.
x0:
The point at which initial conditions are stored.
Generally an integer.
y0:
The initial condition. The proper format for the initial condition
is described in class docstring. To make the function unique,
length of the vector `y0` should be equal to or greater than the
order of differential equation.'

def __init__(self, annihilator, x, x0=0, y0=None):

    self.y0 = y0
    self.x0 = x0
    self.annihilator = annihilator
    self.x = x
