  We present a new free library for Constraint Logic Programming over Finite
Domains, included with the Ciao Prolog system. The library is entirely written
in Prolog, leveraging on Ciao's module system and code transformation
capabilities in order to achieve a highly modular design without compromising
performance. We describe the interface, implementation, and design rationale of
each modular component.
  The library meets several design goals: a high level of modularity, allowing
the individual components to be replaced by different versions;
high-efficiency, being competitive with other FD implementations; a glass-box
approach, so the user can specify new constraints at different levels; and a
Prolog implementation, in order to ease the integration with Ciao's code
analysis components.
  The core is built upon two small libraries which implement integer ranges and
closures. On top of that, a finite domain variable datatype is defined, taking
care of constraint reexecution depending on range changes. These three
libraries form what we call the FD kernel of the library.
  This FD kernel is used in turn to implement several higher-level finite
domain constraints, specified using indexicals. Together with a labeling module
this layer forms what we name \emph{the FD solver}. A final level integrates
the clp(FD) paradigm with our FD solver. This is achieved using attributed
variables and a compiler from the clp(FD) language to the set of constraints
provided by the solver.
  It should be noted that the user of the library is encouraged to work in any
of those levels as seen convenient: from writing a new range module to
enriching the set of FD constraints by writing new indexicals.
