[firedrake] Matrix-free operator on mixed function space

Dear all, I am writing regarding the following problem: https://bitbucket.org/annakalog/buoy2d/src/3ee44ca3db164d8955d363b7d07f62238... I have previously defined a matrix-free operator as described in your documentation to solve a linear system, say A*u = b, where A=M+Q*Q^T. I now need to solve a similar system but in a mixed function space with two unknowns, i.e. A*u + B*v = b, B*u + C*v = 0. Here, matrices B, C have known forms that are similar to a mass matrix. The matrix A is the same as before. How would I solve the above linear system? Kind regards, Anna. -- Dr Anna Kalogirou Research Fellow School of Mathematics University of Leeds http://www1.maths.leeds.ac.uk/~matak/