def __init__(self, indexes, icfn, conversion=<NUM_LIT:1.0>):
self.indexes = indexes<EOL>self.icfn = icfn<EOL>self.conversion = conversion<EOL>
Arguments:
 | ``indexes`` -- The indexes of the atoms in the internal
                  coordinate. The order must be the same as the order
                  of the mandatory arguments of icfn.
 | ``icfn`` -- a function from molmod.ic that can compute the
               internal coordinate and its derivatives.
 | ``conversion`` -- In case the internal coordinate does not have a
                     unit of length, then this conversion factor is
                     used to convert it to a length unit. This way,
                     the Jacobian becomes a dimensionless constant.

All the Jacobian-logic is implemented in this abstract class.