Symbolic computation of a finite element

Hello everyone,

I desire to implement my own finite element, where I made a semi-analytical integration. Using symbolic computation symbolic expressions were found to determine the components of the stiffness matrix. This analytical integration is much more efficient at run time compared with calculations by numerical integration. However, I have been looking for ways to edit the way to calculate the stiffness matrix is made and find nothing. That is, I do not find how to edit the way that this matrix is evaluated. Immensely grateful if someone could guide me to edit files. Or at least see if I can or can not do that edition.

Thanks in advance.