  This paper presents a novel framework for track fitting which is usable in a
wide range of experiments, independent of the specific event topology, detector
setup, or magnetic field arrangement. This goal is achieved through a
completely modular design. Fitting algorithms are implemented as
interchangeable modules. At present, the framework contains a validated Kalman
filter. Track parameterizations and the routines required to extrapolate the
track parameters and their covariance matrices through the experiment are also
implemented as interchangeable modules. Different track parameterizations and
extrapolation routines can be used simultaneously for fitting of the same
physical track. Representations of detector hits are the third modular
ingredient to the framework. The hit dimensionality and orientation of planar
tracking detectors are not restricted. Tracking information from detectors
which do not measure the passage of particles in a fixed physical detector
plane, e.g. drift chambers or TPCs, is used without any simplifications. The
concept is implemented in a light-weight C++ library called GENFIT, which is
available as free software.
