  We derive low-order, inf-sup stable and divergence-free finite element
approximations for the Stokes problem using Worsey-Farin splits in three
dimensions and Powell-Sabin splits in two dimensions. The velocity space simply
consists of continuous, piecewise linear polynomials, where as the pressure
space is a subspace of piecewise constants with weak continuity properties at
singular edges (3D) and singular vertices (2D). We discuss implementation
aspects that arise when coding the pressure space, and in particular, show that
the pressure constraints can be enforced at an algebraic level.
