**test**,

**trial** - symbolic arguments in variational expressions

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)))