DIMEFEM [Williams:89a] is a software layer which enables finite-element calculations to be done with the irregular mesh maintained by DIME. The data objects dealt with by DIMEFEM are finite-element functions (FEFs), which may be scalar or have several components (vector fields), as well as linear, multilinear and nonlinear operators which map these FEFs to numbers. The guiding principle is that interesting physical problems may be expressed in variational terms involving FEFs and operators on them [Bristeau:87a], [Glowinski:84a]. We shall use as an example a Poisson solver.
Poisson's equation is 
, which may also be expressed
variationally as: Find u such that for all v
where the unknown u and the dummy variable v are taken to have the correct boundary conditions. To implement this with DIMEFEM, we first allocate space in each element for the FEFs u and f, then explicitly set f to the desired function. We now define the linear operator L and bilinear operator a as above, and call the linear solver to evaluate u.