NAME

       test, trial - symbolic arguments in variational expressions

DESCRIPTION

       These  classes are used for test and trial functions involved in variational formulations.
       Variational formulations could be specified by expressions, in the spirit of  c++  embeded
       languages.  A variable of the test type represents the formal argument (the test-function)
       in the definition of a linear form, as in:

           geo omega ("circle");
           space Xh (omega, "P1");
           test v(Xh);
           field lh = integrate (omega, 2*v);

       For a bilinear form, the test-function represents its second formal  argument,  while  the
       first one is designed by the trial type:

           trial u(Xh);
           test  v(Xh);
           form m = integrate (omega, u*v),
                a = integrate (omega, dot(grad(u),grad(v)));

       The  field_vf class is abble to represents either test or trial types: it could be used to
       formaly define a differential operator, as:

           D(u) = (grad(u)+trans(grad(u)))